Is the problem that your arm collapses and you land with a thud on your shoulder?
I had this when I first started and I was told to ensure that I extended my arm fully. If you hold your rolling arm out in front of you then extend your hand. You should notice that the extension increases by a a few more inches. You should also see that the shoulder has advanced. If you then go into the roll with this extension you should find that the arm does not collapse. After a while you find you no longer have to do this and will be breakfalling from all angles.