For 11,16 there's a reason the entrance to the level is as wide as it is ;) Getting that one will (if I remember) start a long and complicated path that will eventually get you 13,20.
The star in 7,21 takes advantage of a game mechanic you learn toward the end of the game and uses a very unlikely-looking exit/entrance.
10,11... I don't remember there being a star there, but I finished the game a while back, so my map isn't really helpful for spotting star locations - where are you stuck on that one?