Agreed, I thought a few people had left, but then it seems they've been noting the entire time and in just don't see it.  

 Yep. I thought @ralf left because I haven’t seen his notes in ages but turns out he’s active 

 He’s still here thankfully 💜 

 What can be the cause of this? 

 from what I see in nostrudel you are following more than 800 accounts. 
I was always curious how relays will manage requests to see notes of a large number of accounts. Maybe some relays are silently failing on that or limiting the filter to not all of the people you are following?  

 Yeah I think some notes just never make it through to the client 

 It's the algorithm 

 unironically it is true, even if the algorithm is simply "put posts of all your following in feed in chronological order". 

 Maybe too many pubkeys to request from a single relay, Outbox model helps with this, but still you can run into limits 

 Considering the outbox model, wouldn't that make sense? Not all 800 of your follows post to the same relays to which you have filters. You should be subscribing to their outbox relays right?