A potential easy fix is to do the following:
1) On the side that doesn't seem to close without force or tends to pop open, try lowering the rubber stop. A couple clockwise turns on the hex head (I think its either 13/16 or 15/16) may fix it.
2) Once it is easy to close, check to make sure it is not too low where the clamshell can rattle (pushing rapidly over the latch will determine if there is any play). If there is, then start backing out (counter clockwise) the stop (1/2 turn at a time) until the shell still closes easily and there is no movement (a nice snug fit).
You should be able to adjust it correctly in just a couple minutes.
I noticed the same issue with my car. Variation in adjustments at the factory and temps can cause this.
Hope this helps!