Motorcycle Questions? Ask a Mechanic.
For your ATV to shift into reverse. The left hand brake has the red "R" button you must press down then hold in that brake handle. That will allow you to shift downward into reverse. That button is linked to a cable. So if this is something that just started to happen you will need to see if the cable is broken or working. You may need a person to help assist. You will need to look at the very bottom rear of the engine. Its more noticeable from the right side. There is the cable and that cable is linked to a small arm that is bolted to the engine. You will need to see if that arm is being moved when someone is pulling in the hand brake as described before. If it is not then you likely have a broken cable and it needs replaced. Or the issue is likely down at that arm that is connected to the engine. With looking at these areas you should be able to find the issue. If not let me know what all you have found and we can go from there.