Direct blows toward your device microphone, each device has microphone in its own position. Tablets often have microphone far from where the bubbles emitter is drawn.
If you still are not able to blow any bubble, try to recalibrate it. Open the slider on the left, click the setting button (upper right one) and select "Calibrate microphone".
If you cannot select that command, probably your device has not a microphone.
Anyway you can blow bubbles also with the "blow" button on the bottom right of the screen.