I agree that it is finicky.
I found it necessary to make a few different slight adjustments to the positioning of the connector on the strap before I was able to get it so that black plastic part would lock in.
IIRC, the right side one was easier than the left.
Have you seen the illustrations on page 31 of the Pro-S manual showing the three options?
When I use mine with lens up, I always retract the bellows first.