I believe there is a tiny, very thin piece of rubber (I think it is rubber) that the arm of the bell ringer hits before the round tip on the end strikes the bell, which over time can be worn, hard, or missing. Adhering a thin, tiny piece of rubber, in the correct spot on the metal rest under where the arm lands should correct the weak bell. You can try out a temporary fix, and just use a tiny sliver of electric tape--maybe a few layers thick--and placing it just under where the bell arm rests (the original piece may still be there but may be worn). It is tough to describe, but I had the same issue with my Underwood 3. I tried the tape first, to find the right spot, and then cut a thin strip from the flattest rubber washer I could find, and glued it to the spot. It rings fine now. I think the arm is suppose to hit the rubber spot first, just a hair before the striker hits the bell so that the striker doesn't rest on the bell, and can ring vibrate/ring without being dampened.