Roll over was a bit difficult for us too, but bf was able to get it. Uni only does it one way. Some dogs can to both ways, but Uni can't turn right. I think the trick is to keep the treat close to the shoulder and bring it up around the back. They will try to follow it, causing them to roll over. Of course, many will just turn the head to the other direction. I think you have to do it really slow.
Check out this vid. Uni was about 4 or 5 months in this one. Yes she knows a lot of tricks, but bf literally stayed home from work for like two weeks and worked with her everyday to get to that point. LOL