When you are shielded with PoM on you, the shield takes priority over PoM. PoM only activates when you take damage, which the shield absorbs. Hence, if you're shielded, no damage, and no PoM.
Say you are shielded, and the shield will absorb 100 damage. You are hit for 70, meaning the sheild still holds, and you yourself have not taken damage. PoM will not bounce.
However, if the shield will absorb 100, and you are hit for 120, the shield will break and you will take 20 damage. This will cause a PoM bounce.
If you get stuck not taking damage with a PoM and shield on you, I would recommend right-clicking the shield buff and casting a SW;D to pass it along.