Just as in real life, teams learn how to play against your tactics. In the first season they may try to get a win against you and get it wrong, in the second season they shut up shop and play for a draw. Its now up to you and your skills as a manager to get your team to beat them again...
I am no FM God... all I can suggest is try mixing your tactic up a little, just to give the teams something to think about. Hold a team meeting(in good times and bad). Change your training focus. Try using some younger players, they have passion and desire to impress, older more first choice players get complacent.
Hope it helps...