ESC startup problem with LiPo

Hello everyone. I’m building a drone with the omnibus f4 pro FC and the px4 autopilot. I’m having a bug with the ESCs at startup: when the autopilot is first powered on through usb and then the lipo is connected the ESCs work fine, however when I power up the ESCs and the FC simultaneously the ESCs do not enter in the correct mode and Cannot be armed. Has somebody experienced this issue as well and managed to solve it? The ESCs are cheap 20A hobby wing models, with simple PWM control.