Gowdy states the Judges were not given all of the investigory material.
The State has to show FISA Judge the warrant is effective and producing results. What happens if surveillance results are cherry picked, a distorted picture is created and presented to the Judge, to obtain a FISA extension/renewal???
Generally, nothing other than the warrant issues and surveillance continues.
The FISA court is cover. The surveillance is going on anyway, and probably goes on even when the warrants are rejected. The point of the system is to fool the public. The whole system is shrouded in secrecy, other than the points where a criminal prosecution depends on material obtained under a FISA warrant. Those instances are few and far between. I can't think of a single criminal prosecution that involved a FISA warrant, although maybe there is are terrorism or drug cases that fall into that pattern.
See On the Admissibility of FISA-Derived Evidence in Criminal Prosecutions - 2010
See too The Foreign Intelligence Surveillance Court Of Review Creates A Potential End Run Around Traditional Fourth Amendment Protections - 2002
That second link is good for background understanding of FISA, as well as how it came to be.
In a criminal case, if the warrant is obtained under false pretext, the evidence is tossed, and the aggrieved person can sue for deprivation of civil rights. Tossing evidence is only "valuable" to a criminal defendant, and the FISA law has a statutory defense as long as a warrant exists, regardless of any false pretext.