Hi all,
I had exactly the same problem and thought it was just my plane then all of a sudden i could do harriers, heres how... (my opinion)
when the wing drops try to catch it with the rudder...very simple but it works...be smooth though otherwise the other wing will drop

Another way to get into a harrier (my favourite) is do a stall turn (i think you lot call it a hammerhead) then once the models pointing at the ground pull in full 3D rates and it should go into a harrier/elevator without dropping a wing . Make sure you have height and don't let the speed build up too much in the vertical.

Have fun and good luck, hope this helps a little