Firstly let me say that I dont think his formline is all that bad 7 and up mostly by the look of it so average to above average really. Not certain about this but maybe its your formation and its suitability for the physical aspect of player size, well its a different and maybe better answer than the ones you already have. I have no science to back this up , just a feeling about strikers in this game and the role you ask them to play in a formation. I use a single striker as a target man backed up by 3 attacking midfielders all buzzing around and feeding off him ,well thats the theory anyway ,so I look for physically large 185cm or taller 85kgs or more specimens to play the role. I tried the same thing with much shorter , smaller players as strikers and it wasnt as effective. I have a smaller striker on the bench to bring on if I need to play twin strikers and it seems to go pretty well when I do it.
I am not saying it is the answer but then again it just might be. Would be interesting to know the size and shape of your 3 strikers , might be something in it , might not , who can say what parameters are built into the game engine in regards player size when coupled with formation , I certainly dont know.