If she didn't improve on Three Seeds, but does stabilize on Rehmannia 8, stick with a Kidney tonifying approach. Maybe increase your dose of Reh 8 or try related formulas like Yi Guan Jian. Dang Gui in YGJ increases renal blood flow, too, so perhaps you'll see a greater impact on renal blood flow with it. Don't stop the Rehmannia 8, though. For now, just add in YGJ and see if there is an improvement.
Note that pyelonephritis presents the same (with an azotemia responsive to Rehmannia) but for some reason they only thrive on Rehmannia Six. Rule out pyelonephritis if you haven't already.
Hopefully, this helps!
Steve