I had a very similar problem. My first ATU-10 would complete the tuning cycle but would not achieve an SWR anywhere near 1:1 (it tuned to 5:1 into 50 ohms). I managed to get the seller I bought mine from to replace it, but I had to go through the AliExpress dispute process. The second one that I received does seem to work, but the power reading that it reports is low, about 20-25%.
Back to your no tune problem. I have not had a chance to root cause the issue, but upon inspection of the inside I did note that a wire in the directional coupler was rubbing up against the ground lug screw on the back panel. The wire used is just thin enamel so if that wire has shorted out to ground it would throw off the tuning process. I would start there. I did look at the solder joints on the relays on mine and they seems OK, but the relay pins do not protrude all the way through the circuit board so there can always be a hidden cold solder joint there.
You can download some test firmware at the link below. There is a relay test that will cycle through all of the relays to make sure they are all functioning. If you suspect a bad relay or poorly soldered one, this will help you debug. Be aware that you need to connect the device to your computer using a USB-A to USB-C cable. The computer side needs to be USB-A. If you only have a USB-C to USB-C cable, and USB_C to USB-A adapter should work.
I've read some speculation that some of the relays may have bad contacts. However, I don't know anyone that has confirmed this.
I hope this helps. Try the?AliExpress dispute process if you haven't already.
73,
Scott
AK5SD
Scott Dupuie