What version of WeBWorK and PG are you using? There were some changes to both WW and PG to address some issues with how packages are handled, and it changed how the LimitedPolynomial context is split up. Originally, the macro file was all in one piece, but changes to Perl caused it to be broken into two (a .pl file in pg/macros and a .pm file in pg/lib) in order to avoid some error messages in the log file. Recently, we were able to recombine the two again. So if you updated PG without updating WW, that might be the source of your problem. Alternatively, if you updated both but had overridden ${pg}{modules} in your site configuration file, that might be the problem, as that list no longer needs to include the .pm file.
Davide