Incorrect EVAP purge flow

I got a check engine light 525 miles ago which a local mechanic diagnosed as a faulty purge valve. Apparently, he thought this was the cause of an EVAP system code.
After the work was done, the check engine light was gone, but it's now come back, 525 miles later. As a result, I can't pass the functional part of the California smog test. The smog guy's obd2 scanner identifies the problem as incorrect purge flow.
I think there might be many causes of this scan and wonder how to go about chasing down the source of the problem. I have replaced the gas cap.