This may occur if you have Spotlight indexing disabled on your startup (boot) volume. The App Store uses Spotlight indexing to know when applications have updates available and to process those updates.
To resolve this issue, re-enable Spotlight indexing on your startup volume:
- Open System Preferences from the Apple () menu in the upper-left menu bar.
- Choose Spotlight from the View menu.
- Click the Privacy tab.
- Remove your startup volume from the list by selecting it and clicking the minus (-) icon in the lower-left corner of this window.