Next time you get it apart, check for burrs and file any down (carefully) is all I can offer.
Edit: I've had mine since Oct. and New it was tougher than now, so seems to have worn in some and gotten easier.
Once you have it apart and all cleaned up, just work the top on and off a bunch of times to wear it in some and see how that works out for you, that's another avenue to a possible remedy.