I wish GP would do away with that "B" wing dihedral jig. I haven't found a single experieced flyer that recommends building the wings with that much dihedral. I've been told that the only thing that it does is make ground handling difficult in the slightest of cross breezes. It also seems to limit the plane so much that the pilot gets bored/frustrated quickly, though you say that you can't do loops, and the dihedral shouldn't prevent that. Are you sure you have enough elevator throw? If you're climbing too slowly, you don't have the power to prevent a stall before you get over the top of the loop. This would also make it very difficult to maintain inverted flight, as the PT is going to want a lot of down elevator to maintain level inverted flight. If you have 50 plus flights on this plane, increased throws won't be a problem. Try setting the elevator to maximum, take the plane up several mistakes high and half loop into inverted. If you lose control, just let it roll upright and fly out. You might need to go a little past the top and let the plane lose some altitude to gain enough airspeed to stay inverted, but it's going to pull level rather slowly, so keep the dive as shallow as you can.