If it is a cap that pushes over the lens hood I have a solution as I had a similar problem with my Zeiss 15mm.
Get hold of a roll of Gorilla tape (I tried other tape but the adhesive wasn't strong enough) which you can get for under £10 on EBay. Cut a piece about 2cm x 2cm and stick it near the inner edge of the cap. That will fix the problem. You may need 2 small pieces in which case stick them opposite each other.
Andy