This guide has suggestions: https://3dprinterly.com/ways-how-to-fix-resin-prints-sticking-to-fep-not-build-plate/
I really don't know much on resin printing. I finally tried mine today. I started a print, then realized part way in I forgot supports on mid overhangs so I started over. What I did start did stick at least, so I have yet to encounter your problem in my very limited experience.
I am wondering if anyone else is using a resin printer so I can pick your brain as to why my prints are sticking to the FEP instead of the base.