But the Blues boss insists that he and his coaching staff are doing everything they can to resolve the issue.
Defending set-plays has been Blues' Achilles heel since the start of the season, way before Zola's arrival at the Club,
Friday night's defeat at Sheffield Wednesday, in which Jordan Rhodes put the hosts' ahead when he turned home Ross Wallace's free-kick, was the 18th time Blues had conceded from a set-play this campaign.
"It is a concern and it is one of the areas since I came I said I would address," said Zola.
"We work on that, I promise we work on that.
"Obviously the results are not good but we are working every week, we dedicate a little bit of time for that.
"The point is this is a league where you have a full week every two
"So sometimes you work on certain things but you cannot get everything you want to do because you are always playing. It is not an excuse, please don't get me wrong.
"I am trying to get things working, I am working on things as much as I can - but sometimes it's difficult."