Make sure your horse is going forward, straight, in a good rhythm and balance, and your corners are corners and your straight lines are straight lines! Getting the horse to come round is all about getting him to work up and over his back, and he can only do this if he is going forward enough in the first place. Tucking their noses in might look pretty but clearly you understand the power is from the back not the nose! If he is not used to going forward in a round outline then he will struggle with this at first, but as he gets fitter it will get easier. some horses find it easier to round up when on a circle and bending through their bodies. Get some lessons from a good instructor - they will be able to tell you what to do best!