I had a similar issue, where the top didn't quite come all the way closed. Looking at it from the outside, the canvas top was slipping to the outside of the guide, like in this picture from a post on Planet-9:
Mike Focke has a good DIY to fix this at his Boxster web site:
https://sites.google.com/site/mikefocke2/convertibletopedgedoesnotfallintoguidech