DRV8824: Step frequency can not reach 20K

Dears,

  The motor stop when we set the step pwm frequency reach 20K. there are still output current in H-bridge, The motor makes a sound, but it can't turn.

We try to turn down Risense, but it is doesn't work, do you have a solution, thanks!

e2e.ti.com/.../8078.SCH.pdf