DIAGNOSIS The chain is stretching excessively due to incorrect adjustment, a worn bar, or insufficient lubrication.
FIX Follow the correct tensioning procedure. Loosen the bar nuts. Use the tensioning screw to raise the bar nose until the chain is snug against the bar bottom. Hold the nose up and tighten the bar nuts to 8-10 Nm. The chain should still pull freely by hand. Check bar oil flow. A dry chain heats and stretches.
IF THAT DIDN'T WORK Inspect the bar rails. If they are unevenly worn or have a "burnt" opening at the nose, the chain will not seat properly and will feel loose. Dress the rails with a flat file or replace the bar.
HEADS UP A new chain will have a normal break-in stretch period. You will need to re-tension it after the first 5-10 minutes of use. This is expected.