Open /Applications/Utilities/Keychain Access and make sure that the keychain containing the relevant item(s) is unlocked, and will not lock automatically by selecting Edit, Change Settings for Keychain....
side note: Edit->Change Settings for "System" Keychain is always greyed up (even when unlocked). To change the settings for the System keychain, you need to Right click on keychain -> click "Change Settings for Keychain "System", then Disable Inactivity & Sleep Lock