If you know someone is on here but you can't find their account by searching, ask them by some other method for their account address and paste this into the search box. Their profile will appear, including a "follow" button. More details about how to find addresses here:
This is the equivalent of exchanging phone numbers or email addresses.
(This happens if an account is very new or hasn't posted much, so your server's search hasn't "noticed" it yet.)
@feditips This sounds more like a #fedifailure than a #feditip.
As far as I know the only ways you could have everyone's account searchable on a decentralised federated system are either:
A central directory service, but that would require centralisation which is what the Fediverse is created to avoid. Someone might take the directory over.
or
Every server having all the details of everyone from every other server, but that would be prohibitively expensive and put people off starting servers. (Also, most of the data would never be used.)