My D800E never triggered remote flashes reliably, not in M or TTL mode. Using M, TTL, or "--" mode for the on camera flash made no difference. After its fourth trip to Nikon service to fix the problem I learned today that they are replacing the camera.
The first two times I sent it to Nikon on my own. The next two I did what I should have done the first time and took it to the store where I bought it, demonstrated the problem, and let them deal with Nikon service.
The on camera flash would light up to send commands but the slaves didn't fire and the on camera flash did not contribute any light to the shot. After the third repair commander mode worked 80% of the time.