Why aren't sites being added to Recently Used in LastPass?

If a site is not appearing within Recently Used, enable Track History within your Account Settings.

Log in to LastPass and access your vault by doing either of the following:
- In your web browser toolbar, click the LastPass icon and select Vault or Open My Vault.
- Go to https://lastpass.com/?ac=1 and log in with your email address and master password.
Select Account Settings in the left navigation.
Under Privacy, within the Track History section, check the box for Keep track of login and form fill history option.
Log out of LastPass, then log back in again.

Results:

Your history will now be tracked going forward, and you can visit sites that you'd like to have appear in Recently Used.