In the final season that I have played in 2015 before I quit, I did have 2 very good wingers. One was 6 stars and the other 7 stars. The 6 six-star one was bought one season before the 7-star one was bought. Both performed good enough in that 'last' season. The 7-star winger was a free-kick specialist. He was scoring a lot from free-kicks in first 2 weeks of the season, but slowed down noticeably in the final week or so. In the last few days of the season, they both seemed to have slowed noticeably, especially in important matches.
With all the '****s' that have been happening lately, only Nordeus can tell you what is going on, and even them may not know.
Some people here might come and justify the things and act in such a way as if you are to be blamed for the bad performance of one player or as if their is nothing wrong with the game. But you know what you have observed. If you have observed what you have described above, then this is not normal in terms of football logic. It could be a flaw in this game or it could be that this has intentionally been programmed to be like that. It could also be a coincidence, but if you buy another player and the same thing happens, the probability of it being a coincidence decreases, and if it repeats again and again, for sure it is no longer a coincidence.
There is one thing that I have noticed though. When a team have say a 6/7-star winger or striker, and other players being 4/5-stars or lower, it seems that the good performances are concentrated on that 6/7-star player only. When another 6/7-star player is bought, it seems that as if the distribution of good performances is shared among those two.