Suzigran. Welcome to Gransnet, where all your problems are made smaller!
You should get to grips with Facetime, even if it's just a short chat every week or two. My DD lives in Australia, and has a 2 year old daughter. The 8 hour time difference makes it awkward, but we have a Facetime chat once a week, on her day off from work. Before I go to bed I have a breakfast chat with them, and it's a joy to see my little GD. She shows me her toys, reads me books and I catch up on what they are doing. She recognises me now, and is always pleased to see me. It's a bit sad when she asks me to come to her house, and she once said that 'Grandma lives in the phone', but I go to visit them as often as I can.
My two teenage GC, who returned to England with Mum when their parents split up also Facetime their Dad every Sunday morning, and keep the relationship going between his visits.
I would try to make the visit schedule less rigid, if possible. I think that the relationship with your GC will improve as they get older. It sounds as though they lead very busy lives. Just keep in touch as much as you can, without putting any pressure on them. As the GC get older you will probably find that they have more time for contact with you.
Good luck .