Update
I now have more questions than answers. Took my D7100 and the Nissin flash down to my local camera shop, We set up their 7100 and mine to all the same settings.
On the Nissin flash there is only one mode that the flash will fire in, "SF." So we set the flash to this mode. Set both cameras side by side and firing one 7100 then the other. Both cameras triggered the Nissin and in both cases the flash is before the shutter opens.
Next we pulled out a Nikon SB910. Set the flash selector to remote, group A, channel 1 - same as what we used to trigger the Nissin. All settings in both 7100's identical. My camera will not trigger the SB910 but the stores camera does.
Doing more research. Trully baffled at this point and think my camera and this flash may both have an issue.
You did not say what the camera flash mode was? Manual flash, TTL flash, or Commander?
SF mode simply triggers at the first flash it sees, of any kind. To work (affecting the picture), it has to be a manual flash trigger.
The thing to understand is that there are three systems, and
the triggering camera flash mode MUST match the remote triggered flash mode. Square pegs and round holes.
Whether the Commander can trigger the wireless remote mode can also depend on distance and direct line of sight between them. Your test distance is probably close, so it may not matter. Normally at any distance more than 4 or 5 feet, the remote flash body must be turned so that its sensor aims toward the commander (else it cannot see the commander signals, which are weak, minimum power level).
Workable combinations:
1. Camera Manual flash and remote SF mode should work, easily.
2. Camera TTL flash and remote SD mode should work, easily.
Those two are simple optical slaves, and are only manual flash, so the remote flash must be in Manual flash mode. You set flash power directly in the flash menu. It probably flashes in TTL mode, but you cannot control its power level any way other than its own Manual mode menu.
3. Camera Commander and remote Wireless mode should work (assuming Commander is set to channel 1 group A to match the flash - and that direct line of sight is sufficient to work.) (commander mode can be TTL or manual flash)
No other combination will do anything useful. You don't mention any of these details.
Camera Commander and remote SD or SF probably triggers, but too early, before shutter opens. Flashes, but not in sync, not useful at all to the picture. Seeing a flash is not enough, you also have to judge if it affected the picture.
Remote Wireless mode and camera Manual or TTL ought not to trigger at all (at least if it were a Nikon flash, it would not). Wireless remote mode only understands commander signals, and then only those addressed to channel 1 group A.
The way I read the Nissin manual, getting the flash into wireless mode seems a bit tricky (purple LED)- read that part again.
All settings in both 7100's identical. My camera will not trigger the SB910 but the stores camera does.
I'm hesitant to say anything, I doubt I understand the actual situation... but on the surface, that sounds bad for the camera. Except (if all things are equal) if the commander flash is firing at all, then the intelligence in there is just software, which does not go bad. Hardware goes bad.
Is the SB910 IR triggered or flash triggered?
Not the right question. It is both, triggered by the IR component of the commander flash. But the big deal is that the commander signal is several flashes of binary encoded numeric commands, not just a simple flash.
The SD and SF slaves just see simple flashes (SD ignores first one, assuming that it is TTL preflash). Only the Wireless mode understands the Commanders complicated signals.