Manager Gary Megson blamed Sheffield Wednesday's defending at set-pieces for the 1-0 defeat at home to Stevenage.
Scott Laird scored from a corner just before half-time to put another dent in the Owls' automatic promotion bid following their 2-1 defeat at Exeter at the weekend.
Megson said: "I would say that in our last six games we've cost ourselves a minimum of eight points through not defending at set-pieces.
"We have players who aren't doing their jobs at set-pieces. It's rammed home to them what they need to do and one or two individuals are killing us. I'm not going to name names in public but everyone in our dressing room knows who it is.
"It's a big, big part of football to be able to trust people to do their jobs."