You need to loosen the knob on the bar completely and remove the bar cover and adjusting knob. Clean the inside of the chain cover and inspect the adjusting pin for damage.
When you assemble the chain cover be sure the adjusting pin is going in the hole in the bar. Tighten the bar knob but not tight. Adjust the chain tension knob and be sure it is moving the bar to tighten the chain.
Tighten the chain so the bottom links are running inside the lower groove on the bar. Rotate the chain when adjusting to be sure it will rotate easily.
When you have adjusted the chain properly and made sure the adjusting knob is moving the bar, lower the lock lever to keep the bar knob from loosening.
These instructions are in the assembly section of the owners manual for the saw. You can get an owners manual free at