@burcinc-- Did you use some kind of starch to keep the meat together. I usually use corn starch. Also, after boiling the beef balls, transfer them directly to a large bowl of ice water to harden them. You probably missed this step.
Transferring the beef balls into ice water makes them immediately firm so they will not fall apart. Some people actually dislike beef balls saying that it's springy like a sponge, but that's how they should be.