The bump knob on your string trimmer feeds 1 inch of trimming line from the spool each time you bump the head on the ground. On gas trimmers, there is a bolt on the bottom of the bump feed that engages the spring. When the bump head has damage, or the bolt is worn, you must replace the bump head. Electric trimmers use a bump plate, which may need replacing over time. It's a good idea to remove the bump head and take it to a home improvement centre. Replacing the string trimmer bump head is not difficult once you have the correct part.
Place the trimmer on its side on a flat surface. If you have a gas trimmer, disconnect the spark plug wire from the spark plug by pulling the wire boot off with your fingers.
Grab the trimmer head with one hand and twist the bump head counter-clockwise with your other hand, if you have a gas trimmer. If necessary, you can use a pair of channel lock pliers to turn the bump head.
Grab the bump head cover by the side tabs with your fingers if your trimmer is electric. The tabs connect inside the outer trimmer head. Press in on the tabs and lift the cover off. The line spool has the bump mechanism moulded into the spool. Unless the spool is damaged, the part that likely needs replacing is the spring between the inner and outer spool. Lift the bent spring out of the inner spool.
Take the gas trimmer bump knob or the electric trimmer bump spring to a home improvement centre to purchase its replacement.
Place the large end of the bump spring into the outer spool on the electric trimmer. Place the line spool back in the outer spool and position the bump cover over the line spool.
Push down on the bump cover until the cover tabs lock inside the spool.
Thread the new bump knob onto the trimmer head if you have a gas trimmer. You only need to thread the bump knob hand-tight. Reconnect the spark plug wire to the spark plug.