VS 2012 Application , Access DB + 4 Users. Read Only Error

Hi all,

i have an application that users can search and update records in.

The application and database are stored on a network drive and everyone has full rights to the folder its stored in.

Everything worked perfectly until recently. There has been no changes made.

There are 3 users working with the application most of the time and a 4th now and again.

Every so often now they will all get a error about updating records. And when i open the access DB it has been set in Readonly.

To cure this i have to either:
Get everyone to log out of the application. Duplicated the Database and rename the both to allow for the new one to be used.
or
Get everyone to log out of the application and after 10 minutes its then back to normal.

I don't know why this has started happening.

Things i've already done
> Checked and set any permissions needed in the folder and files.
> Created a new database and added all new data
> Checked all the database connections are closed properly after reading or editing any data in the vb.net app.


Can anyone help out with this.

Thanks in advance
Alex