Some sites split their username field into multiple fields. For example, if a site wants a login number of 123-456-789, they might ask for it in three separate input boxes, and the user is expected to enter "123", "456", and "789" into each input box respectively.
LastPass should work properly with these sites, as long as they're saved from the plugins. However, when they're uploaded from other password managers, such as the password managers built into Firefox or Internet Explorer, they'll often have incomplete data, as the other password manager was unable to save all three login boxes successfully.
So, if you encounter one of these sites, and it was uploaded from another password manager, you can try the following:
1) Remove the site from LastPass
2) Browse to the site
3) Login to the site
4) Save the site using the LastPass notification bar that shows up
You will then hopefully have a working LastPass login for this site.
Examples of such sites include:http://www.aeroplan.com/landing/process.do?lang=E