Allusion in poetry

[busker]

When doesn't it work?  

• When the understanding and enjoyment of the poem depends on the allusion - some readers may not get it.  Also, a poem should be a discrete work, if it does not stand on its own, did you even write it?
  

Eliot has been mentioned a number of times, but I'd like to mention him again. 
Allusion can still work if the reader doesn't 'get' it in its entirety.
I can't think of too many people who'd have read Kyd, Mallarme, and the Upanishads, or can recall passages from Wagner, but that doesn't stop them from enjoying The Wasteland. Here, knowing that there is a reference to something weighty is enough. There might even be a charm in not knowing the exact context of the original, which may be banal once it is no longer enigmatic.