veer wrote:...The database created is saved in a shared folder and every one of us use it.
If you're using an "Embbeded database" among several users simultaneously
from a shared folder, then you're going beyond the design of that particular configuration. In that case, there's no fix because the follow-on users are limited to read-only database access -- which is reflected in various ways such as graying-out fields, controls, etc. In this case, the tri-state checkbox (Null, True, False) is most likely working according to design.
But there is a fix, if you're so inclined. I'm betting you'll get a consistent view (the dark blue bubble for Null checkboxes) when all users have read/write access to the underlying HSQL database. See the links in my signature block for information on migrating to a robust, client-server configuration for multi-user, read/write access.
Either way, make sure you're keeping regular backups because multi-user access to an "Embedded database" is the most risky (data-wise) approach we've seen in the forums.