In the history of FISA (33 years), only about 12 requests have been rejected. So in 33 years only 12 didn't pass the muster.
A rejection rate of .03%
It would be interesting to know the nature of those 12 rejections, e.g. why rejected?
The one (or more) in question concerning the Trump candidacy were supposedly rejected, once or more, before approval was reached. It would be interesting to know what was added or changed, to get approval.
Substance, or merely a different judge?
I have seen huge, complex flow diagrams, but I think the nature of the FISA corruption boils down to a simple few steps and items.