I assume you mean to say that you either have the engine running (so that the alternator is supplying power) or the battery is connected to a battery maintainer so that the ECU can be powered. With that out of the way:

Have you checked the pins of your OBD2 cable for the device to ensure none are bent and there is no corrosion? Also, using some wires, CAREFULLY connect the Live and Chassis Ground pins (of the cable for your tool) to a ~12V power source. The Car icon on your device (bottom of screen) should then have a Green tick. If it doesn't, then you now have an issue. Note: please ensure you have the polarity correct, we don't want to find out what happens if you get it incorrect.