As Leo L has explained why they loose lift the other way to stop it dropping and it takes quite a lot of practice to get right but if you're rolling to the right, as the plane is virtually on it's right hand side feed in left rudder to compensate and keep the nose up, you then ease off rudder as the plane goes inverted and feed in down elevator, as it continues round to be on it's left side ease off down elevator and feed in right rudder and as you start to come back up to level flight come off the rudder. Where you see video's of people doing axial ie flat rolls without loosing height that's exactly what they're doing and where you see rolling circles they're just doing it again and again and again.

To start off and in it's simplest form give a touch of up before you start the roll, then give some down when you're inverted and ease off again as you continue to come back up towards upright and you'll find it helps. When you don't have to think about that any more and it's natural and smooth start introducing the rudder as described above. I had a couple of knee operations at the beginning and middle of last year and whilst recovering I just sat for hours on a Simulator practicing exactly that again and again and again !!!!!