Have an arduino mega with buttons, HC-12, 2.4 non touch tft screen works great when powered over usb, I plug in a barrel jack to a 9V and the screen lights but when I push a button to change display the screen cuts off.. Checked battery volts and its new and shows good.
Where are you getting your 9V from?
I'm guessing that when running from USB, the 5V regulator is bypassed. When running from an external supply, i.e. your 9V, the on board 5V regulator has to provide the power to run your devices and it's probably not capable of doing so.
Its just a 9v battery connected to a barrel plug.. Are you saying the 9V isnt enough power?
One of these?
I don't think that you can get the power you need from that style of battery.
9V is not power, 9V is a voltage, whereas power is measured in watts (i.e. volts x amps).
Yes, one of those.. lol Okay think a 18650 would work?
Two in series (nominal 7.4V) should work. A single 3.7V cell won't get anywhere near enough voltage to the 5V regulator.
But presumably did not check the battery voltage when the button was pressed and the display shut off? 
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