Zwift on my Samsung S5 worked perfectly for weeks. One day, it degenerated into only working intermittently, to the point of it being useless.
It turns out that Android (annoyingly, in my view) automatically battery-optimises any App that has not been used for three days.
Go to Settings, Battery, App optimisation, Detail. If Zwift has a been optimised, tap it, and disable optimisation.