*All* acrylic stamps have the possibility of turning yellow, some do it faster than others, exposure to light is the reason. It does not effect the way your stamps will stamp, I have some from MSE that have turned and they still work great.
If you have CTMH stamps that have gotten "gummy" they are probably from early in the process of making clear stamps for them, I have heard others say this happened to some of the first ones they got, they have changed the "formula" and if you buy CTMH stamps now they will not get gummy on you, promise
Or if you ever soaked your clear stamps in water (like while cleaning) they can swell up and get gummy, don't ever let them sit in water
HTH