If this is the one I'm thinking it is, (hand crank) then slow down and try to be more consistent on the pressure, there's a "feel" you'll pick up. Speed will come but consistency first. Or, put a case in and push hard while trimming, this case will be as short as your going to get one where the trimmer is set at that time. Set your trimmer up this way and push hard, trimming every case.
BTW, I HATE TRIMMING BRASS