Just because your players performed great one season does not mean they will play great the next!, you have to take into account their age, there positions, formation played, the teams your playing against to name just a few!, heres a good example, when you start a new season with a good team and ofton play well smashing some teams then next time you play them it is a lot closer or even a defeat!, this is because they have strengthened there team while most likely you have used the same team as before" yes you have trained them and they have gained a few skill points but this does not seem to matter as you have come up against a more improved team.
I bought a player two seasons ago, a MR with special a special ability, I picked him up on the auction for about 10T, he was young 22 so I though I had a bargain!, I could not have been further from the truth, he got 5and some 4s in the first season!, thinking he was just taking time to settle in I kept him and played him last season for the first 10 games, he got WORSE! he was getting 4s and 3s, even having a SA!, so I got rid of him and bought a 5* MR from the hot prospects at the age of 23 and WAM he got 7s and 8 s every match chipping in with goals so having a SA player doesnt even guarantee good ratings.
I regular change my team, I used to change my first 11 every new season but now have found that only changing 4 or 5 players is enough as long as your team is young!, I will not buy any player who is not a hot prospect, scout or 18,19, I only keep a player about 3 seasons top!, and he will only be in my first 11 two seasons max usually one!, this keeps my team freshened up and I have won the league from season 2 right up to my current season im on 14 which im currently first with maximum points and already 7 points clear at the halfway point!, my point being is if you want to stay on top you have to keep your team fresh meaning better players every season!, you need to pick a formation that works for you and stick to it and keep building you team around it, keep regular tracks on your players stats and as soon as you see a player with constant low stats get rid, as soon as your whole team stars under performing constantly then its time to change 4 or 5 players in key areas!, you need to have a back up plan aswell, If your team is bad first half switch to another formation second half!, this is where having players who can play multiple positions comes in very handy! these simple things I have said make all the difference!, its just knowing when and where to alter things, hope this helps and your results get better bro.