Hi Stefania,
I just made a minor change to make this independent of the form the answer label uses. You can try downloading the new version (rev 6440).
However, I doubt that would be the problem. One thing to check is to make sure that the webserver has permission to read the PGgraders.pl file you placed in the macros directory.
Can't think of anything else at this point.
Arnie
WeBWorK Problems
full_partial_grader
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