hi, my brother bought a walk behind string trimmer from TSC 2 weeks ago. he says the string won't stay on for more than three minutes. is it breaking or being flung off? i used it to string trim around my firewood stacks that are put up on skids. was extra careful to just tickle the skids. hit one that was sticking out about 1 inch and puff, both strings gone and couldn't find any trace of a scrap of the string to see if they broke or came undone and got thrown off. any suggestions from you experts to keep string on for a long time?