Present giving when you are overseas is difficult, In your situation I would be delighted if DS did this for me on a big birthday.
But turn it round. If your son was to have a big birthday, would you know exactly what to give him that he would find really special. I am not talking about money, but do you know exactly what would touch the right spot? Does he know what you, would really like to have as a special present on this birthday. Probably not, so he has shown his care and awareness of your birthday.
You say you spend a lot on your presents to them. How do you know what to give them or do you just choose something that appeals to you that you think that they might like.
Perhaps in those blank Face Time moments you could discuss with your son what the children would like for their birthdays and Christmas, special ideas for your DiL, that might help win her over etc.
I must confess that we as a family celebrate birthdays, but not big time and providing my birthday card arrives within a month, I am quite happy.