Not sure if this will work for everyone but here's the link I found for the Stack versions - https://www.amazon.com/dp/B01NBWXMY5/ref=sr_1_5?s=instant-video&ie=UTF8&qid=1485061427&sr=1-5&keywords=unsolved+mysteries
i had the same problem. see if you can find it on this link and add it to your watch list https://www.amazon.com/dp/B01NBWXMY5/ref=cm_sw_r_an_am_at_ws_us?ie=UTF8
You may be watching the first ones to go up, which were the ones with Dennis Farina. The first season with Robert Stack is definitely there, as I'm on episode 3, with the Beale Cipher!
For anyone trying to find it, here's the link:
Unsolved Mysteries: Original Robert Stack Episodes https://www.amazon.com/dp/B01NBWXMY5/ref=cm_sw_r_an_am_at_ws_us?ie=UTF8