latex preview oddity

[Alex Olivas]

i've recently installed the latex-preview and i get previews for any math
that i now type that didn't exist before.  for example if i had $x=1$ 
somewhere
in the document and retyped it in a different place i get no preview. 
no error
either.  lyx just doesn't bother trying to generate a preview.  however 
if i type $x=2$
(not previously existing in my document) i get a preview.
any ideas?
thanks,
alex.