SQLite database issue in iOS 4

I have an application that has many users who can login and perform insertion, deletion and update operations on a database (SQLite 3.0). It works fine in iOS 3.1.2, but when I tried to run the same application in iOS 4.0, the application functions well for the first time (e.g. for the first time when a first user logs in). When he logs out and a second user logs in, I am not able to perform any insertion or deletion operation开发者_高级运维.

The Xcode doesn't shows any error and the application also never crashes. What can I do?

Just a thought, are you sure you close your database before writing to it? When a database is locked it is readonly.