testers found "the fuel tank inserting system is incapable of providing protection from threat-induced fuel tank explosions during some critical segments of combat missions when the aircraft is most likely to be hit." This system, the report says, is being "redesigned" as a result of the testing. The new system will be tested again to see if it works.
Which presumably means, when descending rapidly from altitude for ground attack, outside air fills the ullage (space above the fuel) through the tank vents faster than the onboard inert gas generating system can either a) produce inert gas, or b) get it into the tank.
To the layman, the 'obvious' solution is to restrict or close the tank vents. Unfortunately this turns the tank into a partial vacuum chamber, making risk of tank implosion an additional hazard for the designer to think about. Its even more serious if your tank walls are wing skins.