You need to determine what the error is first.

If the InGroup() is not working, then the issue is not with the mapping, but rather with the group membership check, and you will need to fix that first.

Just run a simple script with the failing user:
Code:
If InGroup("....whatever group")
  "Am in group" ?
Else
  "Am not in group" ?
EndIf


Once the group check is working you can worry about the drive mapping.

Check that your old and new groups are the same type i.e. Local vs Global.