Replace the spark plugs and try pushing the primer bulb a handful of times if the trimmer won't start.
Clean out a flooded engine by removing the spark plug and rotating the trimmer so the hole is pointed at the ground. Move the choke to the running man symbol and pull the cord 10 to 15 times.
Cut longer grass from the top to the bottom instead of the other way around if the grass is wrapping around the string housing and shaft of the trimmer. Also, try turning it to full throttle for more power.
Lubricate the cutting string with silicone. Press down on the spindle and pull more out or remove it from the spool entirely and rewind it if the line is not advancing.
Locate the idle speed screw on the carburetor. Turn it clockwise if your engine starts, runs and accelerates but won't idle.