Penny,
The Dolphin's runs a self test when first powered on.. it moves a little forward, and then backwards, and then tests the suction motor.. If any of those tests fail, it just stops..
Most likely the brushes in the motors have worn out or there is water in the motor box.
It could also be the drive train, which has no bearings, so the plastic pieces wear and make it hard to move the tracks.
Personally, unless it is very obvious where the problem is located, it is not worth my time to try and repair them...
I have moved on from the Dolphins, because of this.. See this review...
About a month ago I needed a new robot pool cleaner and I contacted Margaret at Marina Pool and Spa. My initial goal was to buy another S200 "style" robot as I have been happy with them so far. Margaret suggested that I might want to look at a new line of robot cleaners that Marina is now...
www.google.com
Thanks,
Jim R.