Seriously though, sometimes i do see either tank joystick not working well. Any thumb position significantly past center will cause the joystick to "let go" and snap back to center. (When it's working fine, pushing my thumb past a stop will leave the joystick at that last stop limit.) Anyway, when it misbehaves, I try to feather the control so I don't go past the "let go" point, which does leave me with very little throw. Then I quit out, force stop the app, and open it again.
Also, do you have a screen protector on the screen? That's been cited to do weird things to some devices, for this game anyway.
Manfred