i've recently introduced my two best friends, and now neither of them talk to me as much anymore

so for context a little while ago my best friend of nine years who i'll call A asked me to introduce them to my best friend of about two years who i'll call B. they hit it off instantly and it was all great, but this happened about six months ago. for a while now, A has been taking multiple days to respond to any attempts to reach out, while i see them daily if not more in calls or games with B.

i'm really genuinely happy that they're getting along so well, but A has this habit of kinda disconnecting from me once they have a new friend, taking the friend with them, which sucks because i am often the one introducing them to people. i want to keep all of my friendships intact but those two were my most meaningful and i feel like they are drifting away. what should i do?