Sometimes if the top hits the ground tip first the tip gets pushed up and causes the bearings to jam up. If this is the case just pull on the tip a little. You'll know if this was the problem, if when you pull you hear a little click. If the bearing themselves are sticking this could be because the grease in them is dried out in which case I recommend soaking them in mineral spirits. You don't have to take them out of the top just put the whole top in mineral spirits deep enough to reach both bearings.
