Have you tried pressing both down and up shift at the same time if it's a model with a thumb lever? Sometimes it jams inside on the older shifters and doing that with a big of finesse should get it working again.
@JimmyAl 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.
Have you tried pressing both down and up shift at the same time if it's a model with a thumb lever? Sometimes it jams inside on the older shifters and doing that with a big of finesse should get it working again.