Focus on perfecting your cartwheel to lunge and handstands. Handstands are a skill that are used in almost all tumbling so the better you are at those, the easier other skills will be. For a cartwheel - start facing forwards on a line and end the cartwheel on the line facing the way you came from (think like doing a cartwheel on a balance beam). Once you've got that down perfect, it will help with the turning to the side and the snap down.
Work on round-offs off of stack mat (hands on the mat, so there is a slight drop-off from where your feet land). This will give you more time to get your chest up and feet together for the landing.
Do you have a video of your cartwheel and round-off? I suspect there are other issues (such as a loose core or bent arms) that are preventing you from snapping your feet together.
