QT: Database is locked Unable to fetch row - sqlite error 5

I'm working on a software that has different threads. Each threads opens its own connection to a SQLITE 3 database.

After using the software for a while, sometimes after 3 minutes, 5 minutes, and sometimes after much more I get the error:

"Database is locked Unable to fetch row" - SQLite error 5

and the queries aren't executed.

What's happening? Do I have to check something programatically in order to see if the database is busy and wait? Shouldn't this be done automatically by the library internally?