If I take the inner reel (the part that string wraps around) off and run trimmer with it off, the bump knob stays put. If I replace the inner reel, the bump knob unscrews itself and comes off. The bump knob screws in clockwise, the spool head spins counterclockwise when trimmer is running. With the inner reel removed there is no pressure from the spring against the bump knob which allows the bump knob to remain in place. With the inner reel in, the spring pushes the inner reel against the bump knob and keeps it from spinning with the rest of the assembly, thereby causing the bump knob to unscrew itself and fall off.
I know someone out there can solve this. Thanks.