Originally Posted by Rowan

Could be the smoothness or lack of on the drop-out on one side or the other. Could even be an angle problem on one side -- as in a slope that isn't part of the flat. And then again, there might be a circular depression where the retainers engage which actually allow tightness unless there is perfect line-up. And look on both sides each of the drop-outs to find the problem.

If making the do-up tight as possible is a solution, but hard to get undone, think about getting a piece of plastic plumbing tubing about six inches in length that will fit over the lever. Carry it in whatever bag you use with your other repair kit.

And yes, I have had one or two problems, the most memorable on my old Fuji Touring that was steel framed and broke the right-hand drop-out on the rear when blown over before the start of a 400km event. It made the event through to the end without me really knowing, except some strange gear shifting issues. I welded it back together, not perfectly, but well enough to have lasted a decade since. A few little issues rose, one of them requiring the horizontal drop-out slot (almost unheard of these days) to be bent back carefully to have parallel status again the one of the left.

So, if your bike has fallen over for some reason, have the parallel nature of the drop-outs been checked?