Is this a 2 stroke trimmer? Sounds like the cylinder is hydraulicing. Meaning fluid build up is stopping the piston. Make sure your Fuel mix (if 2 stroke) is mixed properly. Take the spark plug out and pull the cord several times to get rid of the build up. Make sure your not standing in front of the Spark plug hole because you will get it on you.
David Trowbridge, Service Tech., June 2008