back to belly rolling question...

so when my son was about 4.5 months old he rolled from back to belly towards the right but now that he has hit 5 months this past week he hasnt rolled towards his left. and I'm wondering how long it is suppose to take before they learn both sides or if I need to be concerned with the fact he wont roll that way. we recently have been trying to put a toy that direction for him to reach for but he wont bring his hips with him and he looks like a twizzlers all twisted. so does this eventually happen or do I need to be concerned?