my FaceTime doesn't work?

i have a apple mac air got it about 9 weeks ago for school, me and all my friends just discovered facetime was on the mac so we all decided to set it up and we cant call eachother? it says its calling and we can add contacts, see eachother and it says we've called them, but the calls just dont show? or work? 😟 help anyway shape or form would be great! thanks 😕😝😀🙂😁😎