The seatpost on my bike will not stay where I put it. I had a Pinhead security thingie put on it, and I assume that might be the problem. I've had people with stronger hands than mine tighten it, and it still slides down.

I'm going to check with the LBS that sold me the Pinhead, but wondered if there's anything else that might cause this problem. I weigh 110 pounds so I don't think I'm putting undue pressure on it.