It sounds like your film has taken a set. (Has become curled to acclimate the diameter of the spool it was wound up on.)
With cine film, there are things I would do to counter the set like winding it on a different sized reel or winding with the emulsion in the opposite orientation but I don't think that's possible with undeveloped still film.

What about gently warming the film just before loading it onto the developing reel? Hoping that the base would be a little more flexible and easier to work with. Don't know... Just a guess.