How olds the line, whats the diamiter, and what kind of trimmer is it. Most weed eater heat up at the head because of friction and for certain kinds you have to use a certain size or it will actualy melt it a little.
Because it is heat bonded you dummy! Is that little spooly thingy that the string wraps around slipping? If it is, it's getting real hot in there.
If not, then you might need WD-40 (top shelf in the storage room where you left it last time)
its actually metlte at the end where it breaks off inside then bonds to the other line. I have started winding lesser and looser and that seems to help, gonna hit it with some slip and slide now. They should issue a comic with cleaning instructions( didnt buy new)