Thats exactly what happens to mine Laurel.  The rubber seems to swell and go brittle and then you can’t push them in the syringe.  They don’t glide very well and I’ve been over zealous trying to squeeze med’s out before now and the whole 5ml has shot back in the bottle because it has stuck and when I pushed, the darn thing just jumped all of a sudden to the nil setting.  LOL!