So I am having some trouble with my D500 and SB-700 flash. It flashes in TTL mode but very low. But when I put it in manual, the flash doesn't fire. I tried the flash on rented D850, and it fired in manual mode no problem. The pair has worked before, but now not sure why it is not firing. I have done a firmware upgrade and a factory reset with no joy. Also I have 4 SB700s, all show the same behavior.
Now if I use my SU-800 Commander, it works like 90% of the time at triggering the other flashes.