If you are shooting a .357 there is another problem that can occur. Make sure that you are using flat tip bullets.
My sister in-law had a new .357 Uberti from a top gunsmith and it had the same problem. What she found was that she was using conical shaped bullets and they were jamming in the magazine tube. The standard magazine in a Uberti is the same diameter in a .45 or .357. With a .357, the bullets go into the magazine in a zig-zag pattern and a conical bullet shape can jam the magazine tube. A flat tip bullet will solve this problem.
A number of us, who use .357s, have also put a liner tube inside our magazine tube to keep the rounds in a straight line.