727 transmissions are so complex

I have a stubborn 727 in my 72 Duster that will not shift into 3rd gear. I had a TCI manual valve body put in it and have had it built by what was supposed to one of the best shops in the area. With the car on jack stands, I can go through all the gears and feel positive feedback at all the positions on the indicator. However, soon as I get the car on the ground and drive it, it will not shift into 3rd. Any thoughts? It’s going back to the shop this Friday when I’m off. Thanks!!
Could be the high clutch. That's the weakest link in these trannys. I've had 2 go out. When it shifts from 2 to 3, the kick down band releases and the high clutch is applied. Then you are in 3rd. No high clutch, no 3rd.