My DS, like most, started with a favourite side to roll-his left. At 3 months he learned to roll. Now at 7 months he can still only do everything from one side. He rolls, sits, can nearly pull to a stand-but is absolutely stuck if you put him next to an object to his left as he can only turn this way. He's always favoured this side, before he could sit he slouched every time time to the same side. It's like he's learn one way of doing things and it's all he needs. The more he can do the more reliant he is on turning left.