It is likely your computer’s browser settings are set up in such a way that the cookie associated with registering the computer is not being placed, or is being deleted.
There are a number of reasons why this could be occurring:
1. The browser is set up to not allow cookies to be placed. In this case, no matter how often you select to register the computer, you will be prompted to receive the one-time passcode, because the cookie never makes it onto the computer.
2. The browser is set up to automatically delete cookies each time the browser is closed.
3. The browser is set up to automatically delete cookies after a certain amount of time (daily, weekly, monthly, etc.).
4. There is another program on the computer that is deleting cookies periodically or preventing cookies from be placed.
Allowing cookies to be placed on a computer can allow you to access information more quickly and/or conveniently (with NB Online Banking, for example); however, not all cookies are beneficial so please consider the security implications prior to adjusting your browser settings.