Looking for some help - the agitator moves, but comes off/loose in larger loads of wash. You can easily pull it off the shaft but there is no visible bolt that is missing. The plastic parts which are on the top of the shaft and on the inside of the agitator itself also don't seem broken, but obviously something isn't working (sorry I don't know technical terms). If I purchase the agitator top itself, does it just snap back down onto the shaft? Could the plastic parts just be worn down enough just not to hold the agitator in place? Thanks for your input!