If you are gradually falling after starting your rolls, you are not using enough elevator. To test, roll your plane upside down and get a feel for how much down elevator you need to hold it level. You will need slightly more when rolling. Also, I found it easier to get up to speed on rollers after I had my CG back far enough so the plane did not need any down elevator when inverted. This simplifies things a little because the amount of elevator required should be about the same upright and upside down. Good luck.