Frank, I know the well nuts have what looks like, brass inserts inside. Do you think after the initial torque to tighten, the rubber stretched and then made it loose with a little time or, do you beleive the screws themselves backed out? If it's the screws backing out, you may try some "light" thread lock.
Cycletron, I think I just didn't tighten them up enough when I put them on. I re-tightened and it's more secure, but even with it tight it's not as secure as I'd like.
I'll re-check and see in a couple days, I think I will use your idea and put a little thread lock on them.
Did you get yours on yet?