Whack it with your thumb. That's how i do it. When i do it.
What it does is make use of the fact that the entire mechanism isn't released all at the same time, but with microseconds/micromillimeters delays. Tapping the release will release the first bit, but stop short of anything that should follow.
But there is absolutely no need to do that. You are not going to use the camera that way. So do not bother.
There is no mystery about the release in 500-series Hasselblads.
You push the button, and the rear auxillary shutter should open instantly. Without any delay.
You release the button, and the auxillary shutter should close instantly. Also without any delay.
(The thing that sometimes fools users not yet familiar with the camera is that you therefor need to keep the release button pressed for at least as long as it takes the shutter in the lens to do its business).
If there is a delay, either opening or closing, or both, there is something causing that that needs to be fixed.
I understand correctly that when you cycle the camera in a rapid sequence, it works, but when done with an appreciable interval, it happens again?
That would indicate that it is old grease that is to blame. It will 'loosen up' when moved a bit. But will settle down to being a sticky goo given a bit of time.
A CLA should solve it.