Before I suggest anything to add to what WJ has already correct said, a few questions....
-how often are you seeing these crosses resulting in goals? Is it every single match? Several per match? One every so often?
-what highlights do you watch the match on? If you are not watching on Comprehensive or Full, you may be missing a lot of instances the your players blocking crosses from ever coming in, or winning the ball?
-when your defenders do get beat, do you take the time to analyze why? Did they not react quick enough? Did they react but get muscled or out-maneuvered by a quicker player? Is it a midfielder making a run? Likely, its something a bit different each time but you can start to see what some of the causes are. A great defender who has poor Acceleration can be out-paced over a short distance. A great defender with mediocre Anticipation can still get caught out by that. That said, I do agree with WJ and its not really down to your defenders.
Football is a game of space. That's obvious. But you need to consider how you want to control that space. You cannot control all of it all the time. So decide what you want to concede. Getting the D-line right is pretty vital. Too low and you concede too much space, your team can't close down without losing shape, and attackers can run at you. Too high and you are vulnerable over the top and to thru balls. Having a higher line can help with crosses because you give less space for the wide defender to have to defend the cross, less room for the runners to make runs, and they have to cross it from deeper. But if you have slow defenders, it can cause problems. Again, you are making a choice.
A DM tends to be very helpful as you shield the defenders. Without that, you can end up with 2 central defenders trying to mark 2 strikers and a runner or two coming in free. An Anchor Man is static. A DM is a bit more mobile. A BWM is much more mobile but also tends to go chasing and can leave you exposed. You can have great success with a BWM but it takes the right player and system. You mentioned having a B2B and a CM-A in the midfield. Neither are very defensive. The B2B will help out but often has to cover a ton of ground so may not be positioned well.
Do you have wide besides the full backs? You mentioned using defensive wingers but do you have wingers normally? Even if they are fairly attacking, they can provide some cover and keep your defenders from having two attacking players on their side to deal with. If you use a narrow formation, that's always going to be a drawback. Opposition instructions are great, but you still need players capable of actually doing so effectively.