If you're using a Ryobi cordless battery string trimmer ($99 at Home Depot), you ... a 0.065- or 0.08-inch diameter line. The owner's manual should list the compatible models of spools for your ...
the trimmer line was allowed to wear too short; the string is tangled inside the spool (this is a very common problem); or the engine speed is too slow and must be increased by throttling the engine ...