Stupid question about flash

I've been noticing sometimes when I use flash I don't get consistent light output, and sometimes it's for example, 3 different exposures.

Can someone explain why?

Thanks.

Canon ETTL...not known for absolute consistency of flash exposure. Just is.

If your camera is not tripod mounted, and/or your subject moves a bit within the frame, the way the subject falls across the flash metering zones can have an impact on actual flash amount commanded by the camera to the ETTL flash.

Since you have Nikon, the same issues may apply as for Canon flash.

Is it an "auto", or full manual flash?
Are you SURE that it has completely recharged?

PhotosGuy wrote in post #18151014
Is it an "auto", or full manual flash?
Are you SURE that it has completely recharged?

It's a Nikon SB 5000.. I have it in TTL mode. No I'm not sure if it fully recharged so I guess I probably jumped the gun. I'll pay more attention to that.

Also, I'm pretty sure the batteries are low, I don't know if that affects it.

TTL mode can be fooled just like Auto ISO, P, Tv, & Av etc. can.
Low batteries will extend the time needed to recycle the capacitors.
Even when the "Ready" light comes on, it does not mean that the capacitors are really fully charged. OTOH, is you're only using partial power, that shouldn't matter.