" I keep ending up with a cut with a flat bevel instead of a radius. I can do it with a skew though. Go figure!
Typically it is the left side, for right handed folks, that often is more flat. From what I learned it is because your body gets in the way of rolling to the left. This is why you see a lot of turners (me included) lean to the left to complete the bead. Sometime I will practice with my left hand as everyone says it only takes about an hour for it to become "normal".
It may also help to align your pointer finger with the flat of the blade (skew) or with the flute (gouge). A standard grip probably has your finger about 1:30. If you start the cut with it at 12:00 you can roll the full 90* in a smooth motion. If you start the cut at the 1:30 or so, your wrist will have to twist out of alignment to turn the 90*.
Edit: The above is for rolling to the right. For rolling to the left your finger position would be at 3:00. It is probably exactly the opposite for lefties.