My first question to you would be “Are you using a sand wedge?” I say this because I often work with students who have been attempting these shots using a pitching wedge or other fairway iron. If this is the case with you then the club selection is a major part of the problem.
A sand wedge is designed differently than a fairway iron. The sole of a sand wedge angles downward from the leading edge to the trailing edge. This angled sole, also known as “bounce” prevents the club from digging too far down into the sand. In contrast the sole of pitching wedges and fairway irons angle upward from the leading edge to the trailing edge enabling the club to cut through turf.
The sole design of a pitching wedge or other fairway iron causes it to dig too deeply into the sand leaving too much sand between the club head and the ball. The result is not enough club head speed to propel the sand and ball the required distance.
If you are using a sand wedge and still digging too deep into the sand then a closed club face is to blame. When closing the face of a sand wedge the angled flange is transformed into a digging sole similar to a pitching wedge. To correct this aim the club face to the right of the target while aligning your shoulders to the left and swing along this path.
If the club is still taking too much sand make sure to clear your left hip while swinging through the sand. The club head will now enter the sand trailing behind your hands with the club face open. The open club face increases the “bounce” of the sand wedge creating a shallow path through the sand and sufficient club head speed to displace the sand and the ball. The open face also increases the loft of the wedge so the ball comes out higher and lands softer with less roll.