Anne,
Your system design appears quite good, at least at first glance. Are you using a signal conditioner for the load cell? What are its filtering settings set to? It kind of looks like we could be dealing with too much filtering, since the signal is so smooth.
Also, I see the Actual Force is leading the Target Force. This can be caused by too much Rate Feed Forward.
For higher frequencies, normal tuning doesn’t work. For that, we use adaptive amplitude control, which is described here: https://forum.deltamotion.com/t/adaptive-amplitude-offset-and-phase-control-for-waveforms/208
-Jacob