Here are some common reasons why this might be occurring:
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 Logoff button (see below). Or you can tell RoboForm to Auto-Logoff after a period of inactivity in RoboForm -> Options -> Security.
You have opted to store your Master Password in Windows System Protected Storage
Make sure that the "Don't ask for Master Password" ("Store the Password in System Protected Storage" in v7) option is not enabled in RoboForm -> Options -> Security. Otherwise RoboForm will neither ask you for the Master Password nor be able to Log off locally on each workstation where this option is selected. All your RoboForm User Data will be available for you without MP request, though it will be safely encrypted when e.g. synchronized via RoboForm Everywhere or shared.