Jim Mallinder's men conceded a late try as the Falcons snatched a dramatic 24-22 victory at Franklin's Gardens.
It was a fifth successive league loss for Saints, who have now been beaten in eight of their past nine matches.
And Hartley, whose second-half score had looked set to secure the win, believes simple errors were the undoing for his side.
"We were going to have to earn it and we weren't sharp enough with ball in hand," the skipper said.
"It was one of those games where if we'd done the basics well, we could have won that game.
"As it was, we were in the game with a minute to go and it was really disappointing to concede that try late on.
"We had a few tries disallowed so the rub of the green just didn't go our way.
"I'm not making excuses - that game was there to be won in the late stages."
And he added: "It was just an evening to be brilliant at the basics and we couldn't keep the ball long enough.
"We gave them turnover opportunities and when you can't keep the ball, you invite pressure.
"If we'd just have kept the ball and built pressure, we'd have scored tries.
"There are things to look at."