Hi Techies

My daughter and her family live in Sydney and we communicate via IPads and FaceTime.
sometimes when trying to call her it appears to be calling at the far end but they do not get any indication of an incoming call or any missed calls.
And it's the same the other way as well.
I have checked the settings a few times and can't see anything that would cause it to not to work intermittently, has any one any ideas or experienced the same .