Symbolism is seen as a reaction to Realism, attempting to capture more absolute truths which can only be accessed by indirect methods. Here I'd like to look at how readers and poet position themselves along this Realism-Symbolism dimension.
How do Symbols acquire meaning?
Symbolic thought is of course not restricted to poetry. When someone says "Let x=2", x is a symbol. When someone says "Let x=Pain and y=Tears. Then y/x is a measure of cry-babyism" they are beginning to think symbolically. Here the symbols are introduced explicitly and what they "stand for" is clearly presented. Language uses words as symbols ("candle" to represent a candle for example), but in poetry we're used to reading more into words. In the charged atmosphere of reading a poem, a lit candle represents the fragility of life. These conventional meanings are common in non-poetry, and occasional readers of poetry are good at detecting them (indeed, they might expect a poem to exploit them) but they have trouble with other symbols. Some poems develop their own definition of a symbol - Coleridge's Albatross, for example, or Larkin's Toad - and other poet can allude to these symbols.
When do Readers read symbolically?
Readers are more likely to read symbolically if Realism cues are reduced. A Forest is a place of testing, of challenge, but if the forest is named and described, the urge to symbolise is attenuated. A poem that's bland or obscure or that repeats a word may tempt readers to look beyond literal meanings. It's usually fairly clear to the reader when a poem's trying to develop its own symbol definition, but the meaning of the symbol may not be clear. Blake's mass of symbols may be bewildering, but they're not mistaken for Realism.
Substitution or Augmentation?
Once readers have seen a symbolic interpretation they might need to decide how ornamental the symbolism is. When a symbol is introduced by a simile (e.g. "her eyes were like stars") the first item is real; the rest is "just words". In some more extreme types of symbolism, the real part is suppressed entirely. Elsewhen it's mentioned then discarded, the symbolic meaning taking over.
In the charged atmosphere of a poem, words like "star" and "ladder" are going to be given symbolic meanings even if the poet meant them literally. This readerly urge towards symbolification is exploitable by poets. One of the more common ploys when writing poetry (I use it myself) is to use a title with more than one meaning ("Going Down", say, or "Still Life"), then start the poem with an anecdote where a literal interpretation of the title is followed (going down in a lift, for example, or looking round an art gallery). Then in a punchline the more symbolic interpretations are invoked (depression, etc). The punchline doesn't deny the reality of the anecdote, it adds depth. Similarly in Pound's
The apparition of these faces in the crowd; Petals on a wet, black bough
both images are real enough; the implied symbolism not destroying them. Pound wrote "Go in fear of Abstractions", and "I believe that the proper and perfect symbol is the natural object, that if a man use 'symbols' he must so use them that their symbolic function does not obtrude; so that a sense, and the poetic quality of the passage, is not lost to those who do not understand the symbol as such, to whom, for instance, a hawk is a hawk". Following this principle, one's allowed to use an hour-glass instead of "Time". One can smash the hour-glass instead of "annihilating Time", or even become "Time's assassin", but I suspect one can't end a poem (like Yves Bonnefoy does) with "The trampled snow is the only rose".
Gertrude Stein wrote that "A rose is a rose is a rose". Freud said that sometimes a cigar is just a cigar. William Carlos-William's red wheelbarrow doesn't dissolve into symbol. Neither is it eclipsed. But the reality of the image can be compromised, the poet using reality like Wittgenstein's ladder - to be thrown away after use. A Raven who symbolises Death might start talking. When Wallace Stevens begins "Anecdote of the Jar" with "I placed a jar in Tennessee/ And round it was, upon a hill" we soon realise it's no ordinary jar, it's a "Jar". Another poem of his begins "The lion roars at the enraging desert,/ Reddens the sand with his red-colored noise." where Realism doesn't last long at all.
Once symbolic interpretation isn't ballasted by reality, anything - even even vowels and perfumes - can become pregnant with potential symbolic value, everyday objects becoming invested with the power of Symbol. At that stage, I suspect the common reader gives up, or at least, treats the text as a Rorschach blotch.
Where does all this leave the reader? Inexperienced poetry readers might have trouble with poems that develop their own symbolism. Some readers want to know what's Real and what's Imaginary and don't like boundaries being blurred. Some readers are unable to leave Reality for long, especially if the poem begins as a Realist piece - when the symbolism takes over, these readers don't release their grip on Reality and are likely to describe the text as confused. Coping with something being simultaneously Real and Symbolic shouldn't be difficult (after all, millions of Christians cope with the Trinity) but in practise it requires more flexibility and re-contextualising than many readers can afford. Pound might have a point - poets who follow his dictum might risk readers only seeing the literal meaning, but at least they'll see something.
The final stage in applied maths calculations is to de-symbolize. Symbols may be compact and easy to manipulate. In the midst of calculation it may be impossible to identify what aspect of the Real World is represented by a particular expression, but the conclusion should feed back into reality, even if one needs to extend one's reality to do so. Anti-matter exists, as you can see if you hold your breath long enough under the surface of reality. So do Emperors of Ice-cream.