Defensive Worries

Ok, so im playing as St Albans in the CCL2 and im having a pretty tough time down back at the moment, letting big amounts of goals in. I have tried to diagnose the problems and I cant tell what it is. All of my defenders and my keeper are always getting match scores of around 6.2 which isnt overly bad but they are letting in 3-4 goals a game and even let 6 past in my last game. They are all very good defenders and none of them have bad morale, I cant work it out. Is it possible that my midfield are partly to blame because they are letting too much ball into the penalty area?

Oh and also my central defender won the young player of the month award last month so he cant be doing all that bad.

Are you switching them around too much?

Sometimes I find defenders perform better when they have enough time playing with eachother.

Could it be training? or tactics? (offside trap too often? etc.)

Usually they are the same 4 defenders (4-4-2). I have a feeling that positioning may be playing a part because a lot if the goals are coming from headers and a loose man at the far post all the time.