I’ve been commuting with my converted-to-fixed ‘81 Voyageur lately and something that is driving me nuts is that the seat post will not stay where I clamp it. After a couple rides it slides down about an inch or more. I’ve tightened it as much (or more, really) than I feel comfortable with. It’s a 25.4mm post in a 26.2mm (I think?) hole with a shim to make up the difference. I assume/hope there has to be a simple solution to this that I’m not yet aware of... I appreciate the help!