Re: Old pop-up stopper technology
As Jack said, it's hard to guess without seeing it more closely. Can you take a picture of the pop-up, lever, and if possible, where the lever screws into the back of the drain neck?
Without seeing it, my guess is that either the pop-up notch is worn, the lever is worn, or the two are not properly engaged. For instance, if the pop-up hole is off center (front-wise, more than backward ), it is possible that whomever disassembled the pop-up for cleaning/maintenance didn't reassemble it correctly, resulting in a shorter throw, pop-up not engaged, etc.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!