Why might a previously backed-up item no longer be available in Time Machine?

A previously backed-up item won't be available if your backup volume filled up and Time Machine had to start deleting older items to make room for newer items.

The default folders in a user's home folder are Desktop, Documents, Downloads, Library (hidden), Movies, Music, Pictures, and Public. Optional home folder items include Applications and Sites folders.

Any item downloaded using one of the built-in OS X applications will be marked for quarantine. Third-party applications may not mark downloaded items for quarantine.

To verify a specific service is available from a service provider, first use the Network Utility Ping tab to verify basic connectivity. Then use the Network Utility Port Scan tab to verify the specific service port(s) are open. You should always limit the port scan to the specific ports required for the network service you're testing.