I'm noticing this in the !Fediverse too. Partly, I blame it on my poor installation of GNUsocial, which doesn't reliably receive messages in conversations with people I'm definitely subscribed to. I think this happens mostly with people using Mastodon software. It's not helped that I can't subscribe to my own Fedgroups on http://sn.jonkman.ca/groups . Some people formerly active on GNUsocial instances have moved to Mastodon instances, and I'm no longer reliably receiving their messages. Other people have just left eg. @mk Marjolein and @ErkanYilmaz I'm subscribed to more people than ever, but I miss the old friends.