Defenders static when balls are crossed into my box

I'm in my first season on FM2012 with PSG. So far I'm doing okay but in maybe 30-40% of games I concede 2-3 goals because my defenders bizarrely refuse to intercept crosses or mark a lone striker when the ball is crossed into the box. Quite often I can have three players surrounding a lone opposition striker around the penalty spot, and when the cross comes in the striker will dart to about eight yards out and tap it in, with my three players (usually two CBs and a midfielder) still standing together in a huddle on the penalty spot. Seen in the match engine, it looks absolutely ridiculous.

I have good CBs (Sakho and Lugano) and this happens even if I set one of them to specifically man-mark the lone striker and leave the other one covering with zonal marking.

Note that I am not concerned to reduce the number of crosses into my box, just to get to the bottom of my defenders displaying stupidly ignoring both the movement of opposition players and, seemingly, the fact that the ball has been crossed into the area at all.

I can't see that tactics are the root of the problem, since this is something that's happening in the 10 yards in front of my goal, and following strikers and clearing the ball in this area is basic footballing behaviour that should be governed mostly by the players' abilities.

It may or may not be your tactics. Since it's happening in fewer than half of your matches, it may simply be that you're facing very skillful attackers. Look back over the game rosters where it has happened and check the opposing strikers' atts. Beyond that, try various combinations of a deeper defense line, tighter marking, narrower width and/or ultra defensive mentality for individual players. Also, check on your defenders' concentration. Poor concentration can be a killer on defensive coverage. Poor morale can also cause your guys to freeze on their decisions. I manage lower league teams and I often catch my guys making a reservation for dinner at critical times during a match.

To be honest, the PSG backline isn't really good enough for a team that needs to win a league. You need to look at the mental attributes to see what is wrong with each defender. It would be the 1 area that I would be looking to improve and there are plenty of defenders on the game you can afford that will be good enough.

This happened to me sometimes in my save game with FC Porto.

In one game Otamendi are running to the ball, and with no reason stop, at that time the guest team forward start running catch the ball and score.