Damp got into the reg? Dust got into the reg? PTFE tape got into the reg?
Might be worth a clean out, but opening and closing them can clear blockages (or break them entirely).
Also, worth remembering that, given that CO2 is liquid in the cylinder, the pressure doesn't drop all that much until there's only the gas in the cylinder left (as removing that gas just makes more liquid evaporate, keeping the pressure up for a lot longer than you expect), then it slumps. Annoying but, that's the way it goes, but it may mean that your gauge isn't broken.