Hi Will
@willkia
Firstly, forgive me as I am sure I am repeating myself in regard to giving this advice on one of your other threads "you choose your focus point" ..................so let me say it again!
You should choose the focus point to suit the situation and the type of subject.
In the case of the ladies above and the shot you show of the camera back the reason as I see that they are OOF because is your chosen FP was between them on the back wall! If you had chosen the FP at that vertical position but the one to the right of the one you have used you could have focused on her right eye (or used the left hand one of those 3 Focus Points and focused on the left eye of the shorter lady) this would would have ensured they were both in focus and your composition was maintained.
Can you confirm that you understand what I am describing???
In regard to this one....................I agree about the flash recycle time as mentioned by
@andrewc
But based on the FP choices issue you are struggling as talked about above.
Re: in regard to the OOF issue in the second picture ~ In the initial picture I surmise the focus was on the lady's hair........................but in the second shot she moved and you focused on the window frame in the background.
IMO ~ Focus & Recompose ~ to use the methodology the OP needs to understand the basics first because I fear that as he insists(?) on using f1.8 and at the apparent close in distance of the posted shots he will find other OOF issues that he will not appreciate the reason for it happening and the explanation will more than likely flummox him.
Unfortunately it seems so