Check the motor connection. If there's a connectot about 10" from the axle, make sure it's oushed in all the way to the marked line, not just in tight. If that doesn't fix it, you have to look at the other end of the motor cable that's inside the compartment at the front of the battery. Look in there at the connectors to make sure that they're all tight.
There are other causes, but you need a meter to do some tests to find out more. How did the problem start? That will give a clue to what's wrong.