For all, who can not shoot on Android:
The game has two modes of input:
One with two sets of four buttons. One set at the lower left, the other at the lower right. Shoot with the X button on the right.
The other with either two buttons on the lower right or no buttons. Shoot with the X button on the right. Are the buttons missing in an area, where the health is shown, switch to the other mode and back again. Then the two button should be shown.
To switch between the two modes, use the small red button in the middle at the lower border of the screen.