Make sure “usb accessories” is turned on in “Face ID & passcode”.
The iPad will power down the usb after an hour of the screen locking and this can be a cause for having to wake it up again.
It’s there as a security measure to stop someone plugging in a usb device when locked and having access to your data, but it doesn’t help dj’s that setup early, lock the screen/iPad and come back later, unlock an expect the controller to work right away 
Let’s hope that the issue and you haven’t found another problem that needs a new solution.
