Probably your bolt / clamp. Take it to LBS. they can sort it
or the seat tube on the frame needs reaming
@dancing james started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Probably your bolt / clamp. Take it to LBS. they can sort it