Fullbacks not joining the attack

What Online mode you playing?

I found with the 4-3-3 Attack with them set to go forward they will. Other things that will play part in this is conflicting instructions. Like a deep line with instructions of attack. That will confuse the AI as mixed messages are being given.

Some formations like the 4 2 3 1 Wide for example might be better for what you want to achieve.

If you are like me and you want the FBs to get forward (sometimes unmarked depending on opponents formation) then try that.

OR

If you want to pass with them and press L1/LB that will send them on a run after the pass. There is options and it is finding what suits you.

At the end of the day - what is your style? Are you a keep ball and recycle player - Do you risk losing possession with forced/risky passes in to final third? - Long ball out to the wings?

I have always modelled my football on keep it tight style. Of course sometimes that goes out the window due to conceding an early goal for example (It's going to happen) Then you either go for it or try and keep it tight hoping a chance or 2 will come - Always a risk in whatever style I suppose.

If an opponent is packing 6 or so in to their 18 yard box then patience is the key and not to force it - This is when a FB getting forward can certainly help as it secures an overload and might get you the desired space for that moment. The aim here is to exploit any defensive shape error they will have.

I only play Online Seasons - I will not touch Ultimate Team - That game mode is not tactical and is purely only about "Meta" players and R1 dribbling with spin spin shoot to win. Nothing tactical about that mode.

Also - You need to know your players - One thing EA does not get enough credit for at times and that is building key stat features in to the players so they mimic the RL version at traits.