After the initial 6 points the belts should stay reasonably tight. For the initial 6, the trailing belts will be loose, but they should tighten for each way point.
Try extending the belts, then retract, extend and retract. After the 2nd retraction the belts should be less than one. If they aren’t it indicates there is a problem with the magnets. Either a loose magnet or encoder not reading it properly. You can also try the test under setup, which will indicate if there is a problem with a misssing magnet.
In the serial log. Test checks that the Maslow the motors, encoders and the magnets are visible to the machine. Results in Serial log. If all ok just reports each is OK.