Are you sure your doing it right? For starters, are they even on v17.3? I know older, or even some ~slightly older~ versions loose their sh-- when the repos eventually get the yank. But, as a devotee of Mint Linux which I first read about on this Site um about Two Years ago now? has never given me any problems with its update system. I generally hit this Terminal only a few time each month. because I need to do something that I can't do directly on my Phablet. So I'm almost constantly bombarded with updates I click the Sheild bit, punch out my password and click Ok.
End of story. About the only thing to my chargin that faild to update was the Pale Moon Browser. But, I downloaded that manually and, used its installer. So I can't fault the updates for that one. Having reran the Pale Moon Browser Updater again I was able to get my Update(s). I just kinda wished I had a better wanning then Youtube stating that the Browser was outta date, and would I kindly be interested in installing Chrome.