What are possible reasons for this not to work
The problem here is likely down to poor coupling between the foil, the screen and the wire you are using. I would try again, replacing the foil with a piece of conductive foam, using adhesive copper tape to connect to the foam, and then soldering the other end of the tape to a piece of wire to connect to the relay.

I would also be sure to apply some downward force on the foam in order to deform it slightly, improving coupling with the phone screen.