In small
surf on a longboard you can't stay planted in the same spot on the board and expect to be able maintain trim speed and make sharp turns. If it feels like you are going to stall or lose the wave you need to step forward to pickup speed. If you want to be able to carve a sharp turn you need to step back over the
fins to lift the nose out of the water. To transition between the front and rear of the board you need to be able to walk forwards and backwards, or rather pushing or pulling the board forwards and backward with your feet.
Imagine that you
catch a weak wave, you paddle and pop up, you do a front side bottom turn projecting up the face of the wave by pressing hard down on the board with your toes, crouching, rotating and leaning your upper body in the direction of the turn, and then do a top turn back down the face of the wave by standing up briefly, then crouch down again, pressing hard down on your heels, rotating and leaning your torso in the new direction. At the top of the wave as you come out of your top turn you will lose speed and it might feel like you will stall off the back of wave, take a few quick steps forward to the front of the board and crouch down to get the nose of the board headed back down the face of the wave, if the section in front of you starts to get steeper and faster you can take a few steps to the back of the board with rear foot over the fins (you can do this now since the wave is faster and steeper you wont stall) the nose of your board will lift out of the water making it easier to rotate your board for your next turn.
Sometimes the waves are so weak you can only trim down the line, unless you can
generate your own speed by pumping the board up and down the wave face from standing in the center of the board, once you've built up some speed you can take a few quick steps back over the fins, lifting the nose out of the water and do a carving turn.
Here is a great video: