Fuji x100 and Off Camera Slave Flash
" .. the camera exposure has been taken place before the Di866 has triggered and flashed .. "
That sounds correct. The commonest cause is that the on-board flash is emitting a 'pre-flash'. Try switching the cam's unit strictly to M flash (& if it has any Wireless capability - disable that).
On the Nissin if there is no Pre-flash, use SF.
If you cannot lose the Pre-flash, try SD. Good luck.