My trimmer has impressive power after I adjust the line. About a minute later, the line shrinks to about 3 inches and won't advance and the trimmer diminishes to inadequate power. Are there techniques to winding the line, that alleviate this problem, which has been endemic to string trimmers since the dawn of time? I don't want to have to dismantle trim head 5 times during a session. In looking at bump head, it doesnt seem to have any play, nor capablility of advancing line.