Defensive Blunders

Tactics has never been my forte.

But my Airdrie team has such a soft centre, partly because the defenders are distinctly average, but also most probably because I don't understand the dark arts.

Here is a typical example:

Opposition AMR Young-Coombes has received the ball in his half in a not greatly threatening position, ahead of him ahead of him, my half-back has moved towards the back line to plug the gap as the wing backs recover in transition. Here, Young-Coombes plays one simple ball between my centre back 15 & half back 21, and the blue striker goes in and scores.

This has happened numerous times. In the setup, it certainly looks like my defence is set too wide, there is too large a gap between centre halves, and the striker isn't being marked tightly enough despite my opposition instruction.

Here is my tactical setup:


So my question is, is it simply a case of pulling the defenders narrower when out of possession, and setting the defensive width to "force opposition outside", or do I have to look at this tactic again, and accept that this is the side effect of the defensive shape?