Is it labouring or slipping? A loose clutch will make it seem like it's not working properly. If that's okay, you need to rule out a possible lack of power issue; batteries, connections, solenoid pack, etc. Both the load and control circuits should be checked. The motors are pretty robust but everything fails eventually, it would be the last thing I'd suspect if all else checks out, you can take it into an electrical shop or someone that rebuilds starters, alternators, etc. Replacing the oil in the gearbox is a regular maintenance item that should be done every three years according to the manufacturer if I remember correctly. Check out the manual, there are some maintenance items mentioned that you should look at going through as well as a troubleshooting guide that may steer you in the right direction. if you don't have a manual, download here:
http://www.maxwellmarine.com/downloader.php?i=323