Hi and thank you for your question.
The first reaction that comes to mind when I hear the symptom you are experiencing is the idle is set too high on the ATV.
If you have an inductive tach, which I know is not usually a tool most riders possess, but if you do, check your idle RPMs.
You want it to idle right around 1000 rpms.
If you don't have a tach in your tool box, turn the idle down and observe the results that has on the symptom you are experiencing.
There is no adjustment for the engagement on a CVT style clutch. If it is not functioning properly and the idle is correct, than either a component has worn out or has failed (broken) and needs to be replaced. In either case, it is going to require removing the 2 sheaves from the ATV and inspecting them for wear/damage. Taking the front sheave apart also requires a special tool. If it comes down to that, your best bet is to take it to a dealer to have them take the sheave apart for you, you do your inspection and determine what needs to be replaced, then have them reassemble it for you. I am confident the cost of the tool would far exceed any savings you would realize buying the tool yourself and doing your own work on that component.
Another point to inspect is the faces of the sheaves themselves. They should be perfectly smooth and free of and grooves. Grooves come from wear caused by the belt slipping, usually from getting wet. If you have grooves in the faces of the sheaves, they will need to be replaced as well to get rid of the jumping when put in gear.
Let me know how that goes.