Aerial cartwheels are one of the first airborne skills gymnasts learn, and can be quite frustrating when it takes a while to get. Sure, you can try different take-off techniques (i.e. off a springboard or off a panel mat), and often those can be enough to teach a gymnast the essential detail of what makes a successful aerial cartwheel. But other times, -- and this is true with most skills, especially with younger gymnasts -- repetition just becomes "busy work" and they don't try to understand how the skill works.
Aerials can be done from various approaches: a step, hurdle, jump, chassť, etc., but the most common when first learning the skill is a run into a hurdle. The hurdle, in this case, should be high and should not travel forward much since little advantage is gained with a long hurdle. The aerial itself should not travel far horizontally, and therefore does not need forward momentum.
There are two possible armswings that can be used for this skill, and I've seen both work well. The first, and most common, is to swing the arms from overhead, in front of the body, and down to the sides of the body. The other is swing the arms in the opposite direction, circling from behind, down to the sides of the body, and out in front. (This latter one is commonly used for dive cartwheels.) While either can be used, I personally feel the first one makes the trick easier. When done correctly, this arm swing provides some additional height and rotation.
Another technique that is sometimes difficult to pick up is the importance of pushing off the front leg. This is also where much of the height comes from, and when not enough force is applied here, you don't get enough height to get all the way around. Also, related to this is the swinging motion of the back leg that is already in the air. This motion is also where much of the rotation comes from, and when you don't swing the leg around enough, the aerial is slow in rotation.
Once having gotten around, many gymnasts forget to complete the skill and just worry about getting their feet on the ground. They forget to lift their chest, which adds a little more rotation to help get around. Also, without lifting the chest, the trick looks like it "flops" and has no dynamic.
Play around with the different techniques and see which ones work best for you. Good luck!