its how the game engine is reacting on the players
better is to make notes of the players stats how they are performing each match on their position
and if they are not performing wel in 2 matches try to swith position and try that again if he is not doing wel after a couple of matches and
if it is not working sell him or set him on the bench for 2 matches and try it again if you dont get result sell him or wait a season (5 star youngster)
example i have a defender dc /dl who performs well only on the left dc position (centre) on the flanks and rightcentre he is terrible
and so you have many players that you must tweak on their position and adjust their training