from my experience, pumps wear-out eventually causing the seal to not be as tight on certain tubes than others.

i have replaced my hose on my floor pump 2 times -- once for wear and once for breakage.

recently, i took my pump to lbs because it was not working on various tubes, showing wear, and they suggested a schrader to presta converter to put on pump, allowing me inflate my presta tubes with the schrader valve -- works great!

maybe you could find some type of adapter or replace the hose -- it is cheaper than buying a whole new pump!