Your update channels might be disabled by joomla because internet connection wasn't up once!
Go to:
"Extensions" -> "Extensions manager" menu, then on left-side menu on "Update Sites".
Then select all items in there with the checkbox at top "Check all" and click "Enable" button.
Then click left side "Update", at top click "Purge" then "Find Updates".
Then all your updates you were missing should appear.
Unfortunately, due to this Joomla feature disabling aggressively update channels, the above must be redone from time to time.