I have a 56V Lithium-Ion 15" String Trimmer which I purchased recently. It's working fine except that it won't release the string after tapping the head on the ground (on both concrete and lawn). I tried to feed the string through the hole in the head and wind it up in the direction of the arrow until both ends are about 4-5 inches. Then I tap the head on the ground while in operation but the length of the string still remains the same. I've done this couple of time with the same result. If I manually push the head with my hand, should I expect it to release the string? Does anybody have the same problem?