String Trimmer flattens/blows grass down

Ive used electric string trimmers my whole life, it’s all I needed. Recently moved to a much larger yard so I bought a used Stihl FS-40C. My grass is relatively thin for the most part. When I goto use it, no matter which direction I go, the air flow from the trimmer just flattens the grass down. I was originally using .95 line. Thought maybe lighter line would do the trick so I went down to .8 – still the same result. I have feathered the throttle a little bit and that sometimes works, hard to keep the right throttle to get a consistent cut though. And Ive heard it’s best to run these at full throttle. My electric cuts it fine but even with the biggest battery on it, I still need about 2 full charges and I’ve made the transition to gas everything else in my lawn equipment so I’d like to get the string trimmer going well. Any recommendations of what to do/try? Thanks in advance!