These things are notorious for having mismatched LEDs and shorts or low resistance paths across them. I think many of the ones we buy were thrown out by the manufacturers and that's why they are cheap. If you have a variable power supply and you take the voltage up from zero you will see some LEDs and rows lighting up before others and some LEDs starting up very slowly. That's why they are generally so inefficient. A good led matrix like this should have all the LEDs matched closely with each other to be efficient.