DIAGNOSIS Incorrect chain tensioning. The chain stretches when new and needs frequent re-tensioning during initial use, or the bar nuts aren't tight enough.
FIX Unplug the saw. Loosen the bar nuts, pull the bar tip up to take slack out of the chain, then tighten the nuts while holding tension. The chain should be snug but still pull freely around the bar by hand.
IF THAT DIDN'T WORK Inspect the drive sprocket and bar for excessive wear. A worn sprocket or a damaged bar groove won't hold the chain properly.
HEADS UP Always re-check tension after the first minute of cutting with a new chain. A loose chain is a major safety hazard and can derail or break.