i have osquery agent installed on 11 hosts - 7 linux, 4 windows. 2 linux hosts work fine and always appear in the db. the other hosts, 5 linux and 4 windows, disappear from db and sometimes appear again, randomly. can someone tell why is this happening?

Osquery does not have a central database that hosts could disappear from. Are you asking about some kind of central control server?

yes. fleet db server