After my 6-year-old son tried to go without training wheels on his new bike this summer, I'm realizing he's just not ready. So I went out and bought training wheels. The problem is that I can't get the wheels attached correctly. The part that attaches to the bike is fine, but the wheels either come loose or tighten up too much, depending on the direction of the bolt. Is there something I'm missing? Does it have anything to do with washers? I'd like to be able to fix this myself without another trip to the bike shop (I do love going to the bike shop, but getting his bike there is no fun).