The Tykes have already sealed a top-six spot but missed the chance to climb the table with a sloppy display at Deepdale.
After potential play-off opponents Brentford and Swansea both impressed with victories, Ismael said: “We didn’t put in the performance we wanted and it’s a big disappointment.
“It’s a typical case that when we reach our goal last week the pressure came down and we need to raise that.
“It’s a warning for everyone that we need to refocus and come back to our strength and intensity.
“(The goals) show it was a concentration problem, it seems that the guys switched off this afternoon which I guess is a normal human process.
“Now the good thing is we will refocus and know what we have to do and find the right solution against Norwich.
“The training week was good and the intensity was there but at 3pm in the afternoon the guys switched off.
“We need to come back to our strengths which we did not do. We deserve our position in the league and when you win it becomes an addiction so we’re disappointed.
“We were not able to win so we deserve to stay where we are.”
Ismael’s side have won 13 of their last 18 league matches and will focus on an unlikely Premier League push when the play-offs begin in a fortnight.
There was no sign of that form and confidence as they shipped two goals from poorly-defended set-pieces in Lancashire.
Jordan Storey poached the opener in the 38th minute, poking home his first of the season after Ryan Ledson’s corner caused a goalmouth scramble.
Tom Barkhuizen’s long throws caused problems and Storey turned provider with a flick on for Ched Evans, who smartly finished his fifth Preston goal and second in two games.
There were late chances for Alex Mowatt and substitute Jasper Moon but that flurry was too little too late for Ismael’s side.
Interim head coach Frankie McAvoy has engineered improvement at Preston and this was their third successive victory.
“We knew beforehand that Barnsley were a threat from set-pieces,” said McAvoy.
“We worked hard yesterday on our own set plays because your strength can be your weakness at times.
“We were desperate to do well and the game plan worked exactly how we wanted it to.
“I was given an opportunity here for eight games and this opportunity might never have come up again.
“I didn’t think for a minute we would win four, draw two and lose one of our last seven.
“The players have done me and the coaching staff brilliantly over the seven games I’ve been in charge, I can’t praise them highly enough.”