Lomas, who is still to win a league game since taking over from Kenny Jackett in the summer, said: "I want the lads to believe they are good players, because they have shown that.
"But set plays are massive. Manchester United probably won the title last year because they are the strongest at set plays, when it comes to attacking them and defending set pieces - even with all the quality they have.
"We can't keep trumping up these individual errors. Hopefully we have had our season's worth in the first five matches. But it is down to us to stamp this out.
"I believe in them. I am sure the fans who were here would be asking themselves how we have not won that game of football.
"If we do not cut out the mistakes, we will have a lot more days like this."