DIAGNOSIS The bar is not seated correctly on the bar studs. The tensioning pin is not engaging the hole in the bar.
FIX Loosen the bar nuts completely. Pull the bar tip up to seat it fully against the studs. Hold it there while you hand-tighten the nuts. Then use the tensioning wheel. Finally, fully tighten the bar nuts with the wrench.
IF THAT DIDN'T WORK The bar could be warped or the drive sprocket damaged. Inspect the bar for uneven wear and the sprocket for broken teeth.
HEADS UP Chain tension changes when the saw heats up. Check tension after the first minute of use and re-tighten cold.