M3 (not M2 as originally stated) hex head screws at your local hardware store can make tightening those pullies easier. They replace the little grub screws. Flats on your steppers is critical. As is cable ties (easier to remove when modding).
With all of that ruled out. Use the “pot adjustment” video if you are using standard equipment (GRBL Shield). You’ll be good to go! X-Carve Maintenance/Troubleshooting Videos - Add Your Own! - #4 by RobertA_Rieke