Because there has to be the right amount of a certain ‘lubricant’ within the encoder, it’s important to avoid doing anything which could flush it out, there can be a deterioration over time, with use too - if you search here you may find examples of people ‘servicing’ their encoders back to a better working order, but if doing that was a possibility it may just be simpler to purchase the encoders and swap them out … something for a proficient person to do, not worth the risk if you don’t have that experience
Repairing encoders may be a false economy, it’s possible that a lack of ‘grease’ (and i do NOT mean actual grease, it’s a specific compound that is needed to facilitate the right resistance) could be just one part of the problem, these encoders are not as superior in terms of life expectancy as the ones used on DT DN which are actually called endless potentiometers