what could be the reason why a query like

select * from os_version

would fail (not return any snapshot result) but a query like

SELECT * FROM processes

would succeed (returns snapshot result) on a given device when ran at the same time in a query pack?

Do the results for the

os_version

query eventually appear? Or do you never see the results? If the results eventually appear, your host may be buffering logs (see the osquery documentation here on remote logging buffering).