team/player performance

hi,

i spend a lot of time finding an answer to my question on this (and other) forum and couldnt find...

im level 5 and until this season my team was doing pretty well (2 league titles + c. league title).
at the end of last season and especially this season my team overall performance dropped dramatically, especially
for my "home-grown" key - players which are still 3-5 stars and are fit to play at this level.
Although i sold some and bought new talented players, the new arrivals fail to perform well or just unbalanced
(for ex:6-8-5-7-4). I didn't change anything in particular regarding the teams game strategy and philosophy, i tried signing new contracts with my key players that perform poorly.. nothing helps.
I used to beat teams that are almost 10 points better than mine and now i find myself ridiculously losing to much inferior teams.

For those of you who experienced the same problem, what can be the causes for that?

Thank you!