This is my first time I have been using a drone. I'm using a quadcopter with a pixhawk running PX4 firmware. I'm testing my drone without props connected to a power supply. When I increase the throttle the motors start spinning normally but when I hold the throttle for a time on the same level, some motors stop spinning. But when I change the pitch or roll, they start spinning again. Is this normal? I'm just a beginner.