Second-half goals from James Henry and Leigh Griffiths earned Wolves a ninth win in 11 league games.
Johnson said: "I was really pleased with five or six of our lads. I thought they were magnificent.
"Two or three weren't quite there and that is the problem.
"You come here and five or six of the boys, fantastic. Two or three not so good, bit weak, bit light, but schoolboy second-half and that is what cost us in the second half.
"But generally we played to the game plan and apart from the mistake for the goal, and a couple of mistakes along the way, we were comfortable.
"We frustrated them and we had a few chances. We knew that we could score and Monty (Cristian Montano) has got to stick that away, six yards out a free header, he didn't and that is what I am talking about.
"Those are the sort of things there are the difference between winning and losing a game. It is these minute events in a game.
"Wolves will be dangerous when they get up a head of steam and we knew that.
"But we knew that we would get a couple of chances ourselves and we played some neat tidy football in the first half without really affecting it.
"We were well in the game but the key point was the mishandling for the goal and the events that led to the goal.
"It was going to be a tough task, we were unlucky, the game plan worked to a point barring a couple of mistakes. We now have to push this one away and move on."