1. The RPMs have to be under 1800 for reverse to engage.
2. Yamaha has an updated belt and a shim for the secondary (helped me).
3. When you press the reverse button and it tries to go into reverse but can't try rocking the sled. It will usually engage.
Depending on what year your sled is there is an updated reverse solenoid.
4. If you press the reverse button and nothing happens (no beeping, no light on) it may be the TPS sensor.