Syntax checker stopped working - only for SOME modules/classes

Hello,
I am an experiencing an odd issue with a VB (winforms) project under VS 2008. In SOME modules and classes only, the syntax / error checker seems to have stopped working. In other words, I could type "junk" on a line, like "asdfas", and hit enter, and the IDE does not react (except to put parentheses around it). What it SHOULD do is underline it with the blue squiggly line to indicate that this is an undefined procedure.
In OTHER forms and modules of the same project, the syntax checker is working as expected (I get my squiggly lines).
Any ideas?