AOC 4Vn Pincushion problem function buttons ain't working
 

I've got 3 monitors with pincushion problem and the front function button for pincushion doesn't work,function buttons work for brightness,contrast and other functions but not on pincushion adjustment...all 3 got the same problem but can't seem to find the problematic component..can anybody help me with this one..

thanks in advance..