Canvas redesigned the matching process this summer; see this article in the Community . It is an improvement because it allows you to choose a cross-walk between the Canvas category/group value and the PowerSchool Categories instead of bringing across the PS ones as they stand, but Canvas didn't do much to publicize the new process.
Once we followed the steps described, we were able to get our classes syncing again.
Note too that Canvas is not sending unscored assignments back to PowerTeacher Pro; at least one student score needs to have been entered in Canvas. From other posts it sounds like this change may be impacting being able to sync "housekeeping" assignments that are often just "collected."