Tbh Newcastle have underachieved for years,
IMO. Too many players in the comfort zone who need a good hard kick in the rear.
You need to have a look at what the opposition are doing compared to what they have done against other teams. For example look at their last 5 home games, then look at their games that season against other top-half/top 5 sides, depending on where you are in the league.
If they go 4-4-2 or 4-5-1 attacking (or 4-3-3 as some call it) against most sides, but are 5-3-2 or 4-5-1 defensive against top teams and/or you, then you have to think of ways to defeat their tactics from there.
If you have time, you could always look at all of the default tactics and work from there. Not really what RL managers would do - most sides go between 2-3 tactics at most in a season, so players know just what they are about and feel comfy, confident, etc - but FM treats tactics the same way old-time bank robbers treated safes: keep rotating until you get the right combination.
Be aware as well you might have 'lost' one or two of the players. Best ship them out before you end up losing the entire dressing room.