Each time you remove the brush, you probably swipe it against the bottle's edge to get rid of the excess polish right?. The odd time polish will drip down the sides and dry up only to seal that bottle tight!
The Cure: As a first step, use a jar gripper or wrap a rubber band several times around the cap to give you a better grip. If that doesn't do the trick run hot water over the bottle to loosen the dried polish, then try again.
Tip: When you finish using the polish, use a tiny amount of remover on a tissue to clean around the rim of the bottle and be sure to store the bottle upright. This should prevent you from having to deal with a stuck polish lid in the future!