My dust caps keep falling off the hub and I have no clue why. The first on fell off the day after I installed it. I went to launch the boat which is about 2miles from my house and when I got home one cap was missing. I went to Napa and bought a new set and this past weekend I towed the boat to a lake about 100 miles from my house and lost one on the way and another one on the way home. I have been using the caps from Napa that have a zerk fitting. Does anyone know why this could be happening?