Many players claim they have a "duck". Which means a bad player no matter what his rating.
This is a myth.

I've had good players go bad in the next season. I've had 4 stars play better than 5 stars.

For example, my 5 star striker has 22 goals in 21 games, my 6 star striker has 1 goal in 5 games.

Players often go in and out of form in the real world so it's probably like that. Maybe, change your formation to suit your player.