If I told you to do a cartwheel on the floor, you'd do a cartwheel.
If I told you to do a cartwheel on a floor line, you'd do a cartwheel.
If I told you to do one on a high beam, you'd instantly change it into something that barely resembles either of your first two cartwheels.
Most beginners have this problem of learning a skill on the floor, getting pretty good at it, and working it on the floor line. After a couple thousand cartwheels and having training to move in a specific rhythm it all goes out the window. To many beginners and some intermediates it seems to make more sense to change it all into a frankenwheel as soon as it's time to do it when it's most important to do the one that's been practiced a couple thousand times.
The cartwheel isn't so hard, but not changing it is. Just don't let it change and it will all be there just like on the floor line. Just for fun warm up your cartwheel, do five on the floor line, and on the 6th, 8th, and 10th close your eyes as soon as your first hand touches the floor. Concentrate on just doing the cartwheel, and see where the line is when you open your eyes after your 2nd foot comes to the floor at the end of the skill. Let me know how it turns out, and please don't try it this way on the high beam. I am not Mr. Miyagi.
