it could be that your defenders have a high rating for dirtiness in their hidden attributes. in any case, try changing them from stoppers to defend duty, see how that works for you
Just played my next game against Cowdenbeath and only won 1-0 using this tactic despite having so much posseassion and shots on target. With the squad i have (Most of it is good enough) for the english championship. I should be destroying these teams.