The Blues have been leaking goals at an alarming rate of late, with an inability to secure shut-outs keeping them rooted in a Championship relegation battle.
Injuries have played their part, with back four stability lacking at St Andrew's.
When they did have stability in defence, loan stars Kyle Bartley and Dan Burn played a leading role.
Both men have been taken back by their parent clubs, though, and Clark admits it has been a struggle to fill their boots.
He said in the Birmingham Mail: "I don't think you need to be a rocket scientist to realise that the goals are coming from down the wide areas of the team.
"But, you know, we lost two important players in Bartley and Burn.
"They were magnificent for us in the first half of the season and it was no coincidence that they played together in that 10-game unbeaten run when we took 20 points from 30 and we were on the fringes, seven points from the play-offs.
"Great balance - right-footer, left-footer - physically both of them are aggressive. They bought into the Birmingham way as well, they didn't act like loan players, they acted like Birmingham City players.
"Sometimes that's a problem you have. You start again with a new group with loans and, you know, they don't always consider themselves to be your type of player, they keep thinking of themselves as part of their parent club, and that can cause issues."