The clocks on both devices (i.e. fb's server and your phone) need to be fairly closely synchronised. TOTP effectively encrypts a timestamp using a shared secret

The clocks on both devices (i.e. fb's server and your phone) need to be fairly closely synchronised. TOTP effectively encrypts a timestamp using a shared secret then decrypts it on the other side and checks you're within some bound. If either clock is out it won't work.