DIAGNOSIS The chain is dull or installed backwards. A dull chain creates friction and burns the wood instead of cutting it.
FIX First, verify the chain direction. The cutting teeth on the top of the bar should point toward the tip. If it's on correctly, the self-sharpening feature isn't a substitute for a fully dull chain. You need to manually sharpen it or replace it.
IF THAT DIDN'T WORK Check the bar and chain lubrication. Insufficient oil causes extreme friction. Ensure the oil reservoir is full and the oiler port on the bar isn't clogged with sawdust.
HEADS UP A backwards chain will still spin and seems to work, but it's dangerously ineffective. Always double-check orientation after re-installing.