My Dolphin Premier stopped working after my wife used it last year. It was about 3 months old, so still in warranty, which is 3 years on that model. The robot would run for a few seconds, and then stop. I called Maytronics and they had me check the impeller and it turns out that all of the blades were broken off! Maytronics sent me a replacement impeller and after a 5 minute fix, all was good again.
It turns out that my wife ran the robot without the filter installed--this one has a bottom-loading filter cartridge--and it must have sucked in something hard that broke the blades (metal hair clip? no idea what it was...)
After that I installed the plastic mesh guard over the motor housing that is supposed to be used with the leaf bag, but it fits with the filter cartridge as well. That should prevent large hard objects from getting sucked into the impeller again.
So I learned a lesson from this and also found that I received good service from Maytronics.