I feel a bit mean even asking but I have a very old friend who lives overseas. we used to be very close but have drifted apart a bit and see each other maybe once every 2-3 years when she is in the UK.
I always send her a birthday card and post it early etc so she gets it- can't not really, as her birthday is same day as one of my DCs- but for a few years now she has forgotten mine, even though she has a relative with a b'day near mine- and the most I get is a belated message on FB if I'm lucky.
I find it a bit hurtful as she has gone from phoning me on my b'day years back, to now forgetting it altogether.