Try it with just the film. No extra glass, no sleeves, no nothing.
If you need something to keep the film flat, try putting something on the ends to keep it taught but don't cover the entire strip of film with something transparent. The additional glass and sleeves will create reflections, adding to the problem.
With curly film, I sometimes use strips of glass on the ends or simply use painter's tape. I prefer the latter because handling glass on top of the glass platen of the scanner is a great way to scratch the platen, which evidently is irreversible. Painter's tape is benign and any glue residues (if you leave it on for days....) can be removed with alcohol/white spirit.