You can isolate the three parts of the system by disconnecting the cable from the caliper. Now check the caliper for hanging up by pinching the arms closed with your fingers and then releasing the pressure a little at a time. The arms should maintain the same pressure on your fingers the entire way. If it hangs up for a moment and then returns to full pressure then you're problem in in the caliper or the pads have a lip that is hanging up as suggested above.

If the calipers are good then try pulling the lever, release it and then pull on the far end of the cable. If the cable is still to pull back then it's the brifter arm or the cable. So strip the cable out of the housings and brifter. Re-check the lever to ensure that it's moving well. Study the cable for kinks or frayed strands. Somewhere in one of these three parts of your brake system a problem should become obvious.

The other thought is that if it has been a long time since you last changed out your housings that perhaps the inner liner has worn a groove in the plastic and the cable is being wedged into this groove instead of slipping freely in the usual loose fitting housings. Housings are actually a consumable part just like brake pads and chains.