Not being able to sync'd has to do with the hosting and how the admin set up their instance configuration.
Depending on the software, there is usually a feature for “polling”. This is the part of the #fediverse software where an admin can set how frequent the software will push and pull content and check profiles.
They also check how active an account is, be it local or otherwise, because believe it or not, polling an less or inactive account is also taxing on the server host.
These backend features or settings allows an instance to run. Imagine having 100 users who follow 100 users each. And the server is polling those 100 local users and the 100 users each.
Different fediverse software have done a different way to manage this. Some moved to other database instead of using mySQL. Some are using a different programming language instead of Ruby.
And a lot of other things we will never know about unless we look into their respective source codes.
I'm going to use the overused email analogy here, believe it or not, you don't actually receive every email sent to you. We're talking about legit emails here, they're just lost.
No technology can be perfect. Polling, sync'ing, there will always be something that will not reach you. However, developers and engineers are doing their best to minimise this (like in email land).
The way I see it, people were spoiled by silo or closed-network or closed-garden #SNS. Of course, within your own, it is easier to ensure everything is received. Like, again, in email, sending to the same domain there's a 100% guarantee it will be received. So, people expect it will be the same.
And if you explain the technical side of things, most people will run away and say, “just fix it” or “not ready for primetime”. But they never did that for the web (HTTP/S) and email (SMTP). When Chromium / Google Chrome was very buggy, everyone continued to migrate to it anyway. When developers were calling to kill IE6, corporations still use IE6 and were only forced when Microsoft seriously killed it.
Most people accept the flaws of software and services they recognise and already using but will not accept the flaws of the fediverse. I think that's what we should understand so we can change people's minds and have a better approach.