[Bill_B]

There are two axle sizes in Tetrix (at least.) Motor axle is bigger than the other one. To accommodate this there are two different axle hubs. You can't put the regular hub on the motor, but the motor hub would go onto the other axles but quite loosely. You might not be able to get the set screw on a motor hub to tighten enough on a regular axle. And if you did, the wheel or gear would be off center.

If you are sure you're using the correct hub and axle combination, take the previous poster's advice and check to see that the set screw is being tightened onto the flattened part of the axle.