Come to think about it, when I first had the issue of no signal and hadn't yet figured out the seat warpage issue, I was able to get a signal a few times by switching it among PCIe slots. As recommended for VGA cards, I had originally only focused on blue slot 3, where I was able to get it working the first 2 out of 12 times. Later when I found out about the warpage I also remembered that the CPU control is split between the two CPUs. I never played around enough with the GPU slot location to test seating on CPU2, so it is possible that for me CPU2 was seated okay while CPU1 wasn't.. I'll never know.
That said, there are actually two more related things to try:
1) Even tho you have loosened the screws, there may be a difference between the seat warpage of the respective CPUs that could be exploited - where one CPU may now be seated properly while the other one isn't quite. If that is the case, switching from slot group 1-4 to slot group 5-7 or (vice versa) would tell you that. Note also that CPU1's blue slots (1 & 3) are built to autoswitch from 16x to 8x whenever its black slots (2 & 4) are occupied. I say that because it is possible in the real world, that these two slots react in a more sensitive to marginal connections in the CPU seats.
CPU1 controls slots 1 & 3 (blue, w/ autoswitch), plus slots 2 & 4 (black)
CPU2 controls slots 5 & 7 (blue), plus slot 6 (black)
2) Again even though you loosened the cooler screws, the CPUs may still need re-seating. There are three degrees of effort that you can try to accomplish that end. Thankfully I was successful simply doing "a".:
a. loosen the cooler screws as far as possible without disengaging them from the bracket. Then gently pull the cooler pump away while wiggling it, Then re-tighten marginally.
b. Remove the coolers, reseat the CPUs, replace the coolers, and re-tighten marginally
c. same as "b"; but switch the CPUs
If Mom is too slow repairing the hole in your jeans then, "sverte ræva og gå naken"