Ok, hope this helps someone!
We have about 8 hamsters now and two had stopped working. I knew there was hair wrapped around the wheels, and figured that was causing the problem.
First, we took out the screw for the battery compartment. Under that are four more screws that hold a cover over the motor and wheels compartment. Take those out and you'll see a divided holder; wheels on one side and tiny motor and driveshaft (!) on the other. Reach in and take out the wheel box. Most likely, you'll see even more hair once the cover is off! We used tweezers to just pick and pull until all the hair was off.
In order to put the wheels back in, you'll have to remove the motor too. You'll see how they connect to fit together back into their spot. Take a look at the contacts on the end with the motor. A few had low contacts and didn't touch their receivers inside the hamster shell. Tweezer these up higher.
Take a look at the P shaped receivers that fit the contacts - several of ours had sunk down too far and weren't connecting. Tweezers can fix this if you can reach them, otherwise thread floss through and tug them back into place.
My DS15 had a blast watching me take the first one apart, and then the old Tom Sawyer in me let the Huck Finn in him do the remaining 7 hamsters, zealously! It sounds more complicated to explain than it is to do.
Now our hamsters run FAST, still on their original batteries. This is just something we'll probably have to do from time to time to keep them running. Returning them as defective won't help the problem - - it's simply hair getting wrapped around the wheels which in my house will recur.
Have fun!