A lot of people say the AI "learns" your tactic and it suddenly becomes hard to win, but I think that's a huge exaggeration. The scouting reports you get before each match that suggest playing a high defensive line or a slow tempo or whatever are probably more in line with the tactical changes other teams make when they face you. Sometimes changing up your tactic can have positive results (for a number of different reasons, not just that the other clubs "figured you out") but that doesn't mean you should make a new tactic every time you lose a couple matches.
There are other factors that I think are more important: player form, jadedness, morale, tendency to struggle in certain situations (the things they tell you in backroom advice), players react badly to pressure or mind games, team talks, players not motivated because there is no competition for first team, the list goes on.
I suggest you play through it if you have been happy with your tactic up until this point. Check out team talk guides, get your player morale back up, criticize players with poor form and drop them from your first team (but praise for good performances too), have your captain talk to your squad about run of poor form if the option is available. If you don't have a deep squad, hire new players who can challenge for first team spots when your preferred XI isn't putting in the effort. Watch out for player marking mismatches (especially between slow defenders and fast poacher type strikers). Basically just work on improving squad management before you start losing confidence in a tactic that worked for you before. Good luck!
