On this lift....are the drive wheels hydraulically driven or electrically?
You say the hydraulic functions work,if the wheels are hydraulically driven check the following:

Inspect center post.....
Remove hoses at V1 and V2 ports of Dual Counterbalance Valve. Plug hoses and cap valve. Install a 5000 psi pressure test gauge to Pump Test Port While activating drive control function, check pressure. Should read 2400 psi (while activating function)

Inspect drive control valve...
Remove hoses from drive section of Control Valve. Plug the hoses and cap valve. Install a 5000 psi pressure gauge to Pump Test Port. While activating drive control function, check pressure. Should read 2400 psi (while activating function)

All this is from the troubleshooting manual in snorkellifts website