I had a 421b then changed it to the 401 and found the axis for hitting the sensor was much smaller, sometines I have to "reach" around from my chair to change a channel. My pronto hits it everytime due to it's powerful transmitters but that remote isn't always handy. Try a uhf attachment for your 2100 (made for any brand of remote). I've seen them at www.smarthome.com and some users here have found ways to disasemble the device and place it within the body of a pronto. I'm sure you could find a way for it to work with your remote.

Good luck