My first thought given the info is that the cable has too much friction somewhere along its path, a sure sign of fraying inner cables. Can you detach the cable from the der, or a length of housing from a frame stop, and then pull the cable out of the lever a couple of inches? The #1 location for a shift cable to break is just inside the lever and out of sight unless one does what I suggest to expose that portion of the inner cable. How often do you replace the shift cables? For some riders, once a year is suggested. For others every few years. Mileage, conditions and rider dependent but replacing a cable is cheap insurance from problems happening during a ride. Andy