i had that problem for a long time, finally it clicked after ten years of going just a few inches under and being pushed back. (i'm 5'8 120 ride a 6') I practically do a handstand over the front of the board while kicking my back foot up way over my head, wing it way up there...way more than seems appropriate. you'll need all your 90 something pounds directed over the nose to sink it enough. i think being smaller things need to be more exaggerated. when you're under as far as it will go pull it in towards your chest hard with your hands while kicking the tail down and that'll scoop you in a long arc. hope that helps.
Look on youtube for videos on duck diving.....Note the hand positioning on the board. I see a lot of people try to duck dive a board to far down from the nose to really push the board under where you can angle down below the wave.
Momentum, you need lots of it going into a duckdive if you want to come out the other end with any forward prorgress. Paddle really hard up until you get about 3-5 feet from the face, grip the rails of the nose, push down as hard as you can like doing a handstand / push up, time it so that you are fully under water before you get crushed by the whitewater. Foot on the bottom with some push from your leg as the wave is mowing you down will propel you out the back of the wave. If you start getting dragged back just start paddling ASAP without losing your board. Lastly, eat something and get some weight to push that board down better HAHA