Well, if this is a sim, then it should simulate real life. Looking at clubs with long term managers, they don't really change their approach or formation, they have three options depending on whome they face, and what players they have available. Most long term coaches have an idea of how football should be played, and they stick with that, and just get players that fit their vision. I doubt the game has some sort of algorithm to screw up your tactic, it's just that you need proper players for your tactic...and I mean proper players, not wonderkids but players that fit at least one role and duty in your tactic, preferably more positions.