I noticed my middle blade was not mowing. After trying to tighten the blade bolt several times on the tractor I decided I'd have to remove the deck. With the deck off and up on it's side I attempted to tighten it again. It just wont tighten enough to hold the blade. It turns freely even after being tightened with an impact wrench. The threads are fine and the proper washers are in place. The two outer blades function great. The gearbox must be okay because there is no noise or rattle coming from it and it turns the other two blades. The shaft and bolt turn with the other two blades but the middle blade floats like it's not properly tightened. I don't relish taking it to the dealer because it's in another town and loading it on my truck is quite a pain. Any suggestions?