Layer shifting: belts, speed and slack

Shifting means the head lost its place mid-print. Either something mechanical slipped, or the motors were asked for more than they could deliver.

Same layer every time?

Then it is the file, not the machine. Step to that layer in the slicer preview and look for a thin island or sudden overhang.

Random height?

• Pluck each belt — you want a low note, not a dull thud
• Check pulley grub screws are tight on the motor shafts
• Push the head by hand: notchy movement means dirty or dry rails
• Drop speed 30% as a test — if it stops, it was acceleration

After any belt work

Re-run your machine's vibration calibration. Changing belt tension changes the frequency it is compensating for, so the old numbers are now wrong.