Is it normal for a baby to only roll over on their left side?

Is it normal for a baby to only roll over on their left side? He started rolling over at 3.5 months, now he's 5 months old, and he should have been able to roll over in both directions by now. If lying on his back, he rolls over with his left elbow, but if you put him on his right side, he tries but can't and falls back onto his back.