Why isn't RoboForm asking for my Master Password?

You are still logged in to RoboForm

When you enter Master Password it is cached in memory so that you do not have to enter it all the time. You can tell RoboForm to "forget" the cached password by clicking the Log out button, or you can tell RoboForm to Auto-Logout after a period of inactivity. To adjust the auto-logout, click the RoboForm icon >> Options >> Security >> check "Auto-Logout on User Switch or Lock Workstation" and set the amount of time as desired. 

You have opted to store your Master Password in Windows System Protected Storage

Disable the "Don't ask for Master Password" ("Store the Password in System Protected Storage" in v7) option. To do so, click the RoboForm icon >> Options >> Security >> uncheck the "Do not ask for Master Password" box.