Defending

Just had a really bad start to a season after 4 consecutive title challenges and I'm putting it down to defence. The players themselves are very good (high tackling, marking, positioning, anticipation, speed, agility, jumping etc.), but for some reason they often seem far too wide and strikers just dribble through the middle of them. Also very often the wrong one goes out to challenge a player, resulting in both centre backs on the same side, simple pass across and they're in on the keeper. This is how pretty much every goal has gone in against me, what is going on, it's not happened before. Should I try zonal marking or man marking? What could need changing? Also I want them to cover each other, I'm not playing offside trap, but when one defender presses, the other one does nothing, how do I get him to come across and cover any players the presser has left with space?

Best thing to do is post up a screenshot of your tactics so we get an idea what your trying to do.