The head coach saw his side surrender a 31-12 lead with 15 minutes to go, conceding four times during that period.
Mike Haywood's score proved to be the match-winner, sending Saints into the Gallagher Premiership top four in the process.
And Hatley said: "At 31-12 there shouldn't be a way back into the game for them.
"I thought we'd controlled the tempo really well and at 31-12 they shouldn't even get a bonus point away from home.
"It's the common theme over the last three or four weeks that we've let teams back in and it's not good enough.
"We lost Josh Bayliss to injury and we couldn't replace him, then Jaco Coetzee has been put in the bin so we had 13 men and there was always going to be a little bit of panic.
"We worked our way back to two metres from their line at 31-31 but then we were inaccurate at the ruck.
"We gave ourselves a chance after all of that and then didn't complete it so it's very frustrating.
"There were some good bits but that's the issue - the good bits are very good but the drop off is stark.
"We need to see out longer periods in the game and we don't. That's the big thing we haven't got right over the course of the season.
"When we've been good we've been very good, and when we haven't been good we've been very bad.
"That drop off just can't be that stark."
The tide turned in the game after a lengthy stoppage as Saints lock Brandon Nansen was stretchered off.
And Hatley said: "The impact of the bench is massively important.
"After that break for the injury, we lost the put-in at the scrum and it unfolded from there.
"We want impact from the bench but we didn't get that as much.
"Orlando Bailey was very good when he came on but we need more from our set piece."