TTL with D750 and YS-D2

Happy Easter holidays altogether!

I have a Problem, where I need your advice: I put an artificial test scenario together in my livingroom to test the effect of diffusers. Flashes and camera were set to TTL, connection via fibre.

So having shot several photos with exact identical settings (all manual, ISO fixed) available light and position, the results differ dramatically - sometimes too dark, sometimes very bright....can this be TTL problems? Do I need something like a pre flash? I switched off red eye effect, but nothing more....

Any advice?

Regards, Sascha

in addition some questions:

- I put the settings of the flash to DS-TTL (light blue)...or does only TTL Slave work (dark blue) work?

- what is TTL Slave in comparison to manual mode?

- if manual mode: with or without Pre Flash for D750?

Regards, Sascha