Paul Tisdale lamented his Exeter side's inability to defend set-pieces which proved costly in their 2-0 loss against Chesterfield.
Ollie Banks set the Spireites on their way by heading in from a corner just two minutes after half-time and Marc Richards crowned the win with a fine second.
And having also conceded from set-pieces in previous home games against Bury and Burton Tisdale was frustrated.
"I'm disappointed with the result and very disappointed to concede a corner a minute after half-time," he said.
"That is three out of the last four games now, so there is something concentration-wise and there is some mental toughness we have to address.
"We have been very good at defending them for the past 12 months, but it has been recently and they are all goals when the game is 0-0 or 1-1, so I'm very disappointed about that.
"I was very happy with the way we played in the first half, everyone contributed and we looked a really fluid team and tough.
"We moved the ball about in awful conditions, we didn't make all the right passes, but then, a minute after half-time, it all counts for nothing if you don't do your job.
"It is something for me to address and the rest of it then becomes frustrating and because we are losing, people focus on the things we did wrong.
"We didn't take our moments in the first half when we had four or five opportunities in and around their box, but they didn't really amount to anything."