Remember that the backroll is an edging and line tension trick. You are coming in with speed but letting off your edge and not riding all the way up the wake.
Take a 15ft progressive edge into the wake but the next time you try it edge back and away from the boat moving the handle towards your leading aka front hip. You will feel the line tension speed you up and the flip itself will whip you right around.
Again remember to really edge all the way up back and away from the wake.
Also there are plenty of training videos out there too.