Coming up out of posture during the downswing is a common fault created by a lack of strength in the glutes (buttocks), try stretching and strengthening them by lying on the ground with your knees bent and lifting your hips and buttocks off the ground for ten seconds
With a buried lie you must use your sand wedge as a digger to get below the bottom of the ball so square the clubface to dig into the sand and create a more downward swing arc
Look up and down at least twice to burn an impression of the line to the target in your mind