FaceTime and iPad - can FaceTime with some apple devices and not others

Hello...my daughter has an iPad model no. MF432X/A (if that's any help at all) - may be iPad 2. It's several years old now.

She is able to easily FaceTime from her iPad some friends with Apple devices, but not others also with Apple devices. Those she can't contact on her iPad can be contacted through FaceTime on my iPhone.

It seems a bit strange and random that some are contactable from the iPhone but not the iPad. I'm probably missing something but can't work out where the problem lies.

Any suggestions would be gratefully received.

Thanks