Saints boss Chris Boyd admits his side were 'pretty flat' in their 27-16 defeat at Wasps on Sunday afternoon.
The black, green and gold failed to score a single point during the second half as they failed to back up their big win against Exeter Chiefs.
Saints had produced a fine showing against the Chiefs eight days earlier, but they lacked the energy and creativity they displayed on that occasion.
And after going in one point down at the break at the Ricoh Arena, they were eventually unable to take anything from the game as Willie Le Roux scored during the second period.
"It's two steps forward and one back for us," Boyd said.
"It was a pretty flat performance from us. We need to go back and see how we can avoid that.
"For us to put in a performance like that is disappointing. To be honest, we didn't deserve to win that game.
"Our preparation was good but I've been coaching for a long time and I've never worked out how to predict a performance.
"We just didn't seem to have that edge that was there against Exeter."
Saints had gone into the weekend sitting sixth in the Gallagher Premiership standings, but they have now dropped to ninth in a hugely competitive league.
And Boyd said: "One of the reasons there is a lack of consistency across the league is because it's so tight.
"You would be hard pushed to say that anybody is having a consistent run.
"Sale have done really well but there's not many teams that are putting four or five performances together and that's because there's such an equality about it.
"If you're two per cent off the game you're going to pay for it and we were probably five per cent off it today."
South African full-back Le Roux registered the game's key try after the break, picking the ball up after Saints had failed to successfully play from behind their own try line.
But Boyd feels it should never have got as far as the score as the replays appeared to show that scrum-half Cobus Reinach grounded the ball in the in-goal area before offloading it.
"The only thing I'm thinking is that Cobus clearly grounded the ball and I'm not quite sure why they didn't go to the grounding," Boyd said.
"They had to decide whether it was carried back and if it was it was a five-metre scrum, if it wasn't carried back it was a 22 drop out.
"I'm not sure you can continue to play on if you've grounded the ball.
"That incident and the ball slipping out of Luther's hands under the posts at the end were just two of the moments that were critical and went against us."