When I stack on the ground I will sometimes put a treat on the ground about 18" to 24" in front of my dog. This will make them arch their neck & it will tightend the back muscles & maybe bring the tail up.
I would reccomend looking into the reason she does not hold her tail up. A lowset tail is structual.
A drooping tail can be a physical problem. When my dogs start relaxing & dropping their tails I have an adjustment done. Voile! Tail is where it should be. An Orthipedic Adjustment can work wonders. A dog can be "out of Aliegnment & we don't realize it until they are realiegned! Find a Chiropractor or someone who does V.O.M..
Best of luck
Becky & the Ninjas