Try moving it more. Two smidgens. Three smidgens. Four smidgens. Move the shifter enough that the cage is centered relative to the chain. (trimming it means centering it or at least moving it enough that the chain isn't hitting the side plates of the der cage)
It could also be that the cage is set at a sharp angle to the chain line where you just can't trim it.
If you can't figure it out and do take it to the LBS, have them show you exactly what is going on and exactly how to fix it. Knowledge is power!
