“They were all sitting around at 11 PSI. The threshold is usually 13.5,” the MassLive source explained. “(The Patriots) told the refs they were a little under inflated or they felt that way. At halftime, they confirmed and obviously put air in them.”
Patriots-Chiefs Deflategate Part 2

The Patriots apparently noticed that something was wrong with the balls after Chiefs kicker Harrison Butker saw his opening kickoff land on the 3-yard line when almost 90 percent of his kickoffs end up as touchbacks. It became more clear that the balls were not to the standard weight when the trajectory and hang time of the balls appeared different. The suspicions only intensified after both Butker and Patriots kicker Chad Ryland missed field goals in the first half.
It's worth noting that the footballs traveled further in the second half after the report was made and fixed. The referees checked the balls, confirming that it's underinflated by two pounds.
And for those wondering why the two teams didn't notice that the balls were deflated prior to kickoff, the MassLive report highlighted that teams aren't allowed to use the kicking balls before the game. The referees were also supposed to weigh the balls before the game.
The good thing is the issue didn't seem to affect the result of the game, especially with the Chiefs dominating for the 27-17 win. Still, it's a lesson learned for NFL officials since they can't really afford to make key misses like that.