I would either use a shoulder guard or get a different sheet. With it rubbing on only one side, you have to think what its doing to her back/shoulder/pressure points/muscles etc. It may end up making her need a massage/chiro appointment.
What brand are you currently using? Maybe try to find one you can better adjust. I really like the one jaydee posted, although I've never used it. I like how its really free in the shoulder. I also use thicker ones (instead of that fine mesh) I've had success with BigD, Shedrow, Glover and Kensington flysheets. Out of the three I like the Glover one more though, its a big more lighter than the others and conforms to their body better (more flex to it) The only issue is I find its not AS durable although it depends how your horse is with blankets. My guys rub on everything, have trees in their pasture and like to play fight. So far its held up well this year and I havent done a single repair on it although there are some small frays.