A couple of quick fixes to help with the fat or chunked shot. Ball position can play a big factor. If the ball is too far forward often times players can bottom out the swing before impact. If ball position is too far back players can get steep and hit behind it as well. Play around with ball position and see if that leads to better contact.
The biggest factor that I see is a decrease is speed. When a player starts to lose speed close to impact the weight of the club will take over and the club bottoms out well before impact. The fastest part of the swing always has to be impact. This is true for every shot from a putt to a full swing and everything between. Work on a slower back swing and more speed at impact.