If this is so then your change over valve in not changing over…
You can verify this by pinching the fuel line from the corresponding tank, before the pump, if the pump starts to make cavitating noises then you know it’s drawing from that tank.
This off course doesn’t include the possibility that it’s drawing fuel from both tanks… a bench test would be more conclusive.
I had a hard time with this valve and after several attempts to make it work I gave up and went with a different solution.