I have the same trimmer and have never had this problem. Use the auto winding feature: cut two 13 ft pieces of line, stick the ends into the holes in the head. You have to make sure the arrows are lined up on the head when you do this. Then turn the bump knob clockwise, winding the line in as you turn. Keep turning it until you have about five inches of line left sticking out. If one is longer than the other, trim one to even it out. Then get to trimming. You should rarely have to open the head up if you use this feature. If you continue to have trouble, visit your dealer and have him give you a quick hands on lesson on how to do this.
Mark
July 2007