Facetime won't work Mac-to-Mac

I am on a MacBook Pro running 10.9.5, using Facetime 3.0, and it works fine when placing or receiving calls with iPhones. But I have two people who have Macs (a Mini and a MacBook Pro, both running Snow Leopard) and I can't get it to work with them, sending or receiving. The calls seem to initiate and say "connecting," but they never do. What could be the problem, given that all three computers work fine when calling or receiving calls from iPhones?