Set piece problems

Hi. I am managing Thurrock in the Blue Square South. Every year I choose this league to start the management and especially the team who has the media prediction of beeing last.

Now after 3 seasons the team is getting better and I achieve a safe-mid table position easily. But I am having huge problems with conceeding goals after set pieces especially corners. Half of my conceeded goals are after set pieces. I train high defending set pieces and I gave player instructions in order to mark the opponents. My central defenders mark the tall players etc....Still I conceede goals from the opposition central defenders. My midfielders are bad at marking but how can I improve the situation? How can I defend, what can I do? You need to realize that in the sixth league we don't find defenders with good marking attributes. Still maybe someone can give me a little help.
Thanks a lot.
Cheers

well you need to assign your players with the right role when it comes to marking.

usually on the far post ive noticed u never need someone who can head good, so i put a player with shitty heading and marking on the far post. near post you need someone with abit of strength and heading probably because some teams put someone on near post, even though its often a big defender so one of your defenders will mark him.

but otherwise its simply marking and heading which is important. usually if i got too many shit players at marking and heading i just put more players to go back.