I know that this is an old thread, but I would like to add some, I think, important detail.
Recently I was able to make context-based reminders with Siri with varying success. In some cases, no matter what method I tried - with date+time, with location, without them, etc., but nothing worked while in some others, Siri made the reminder without a hitch...
Today while at work, when Siri again didn't "want" to create a reminder for the e-mail I was reading, I realized that in all such cases I was connected to the office WiFi network and that in all cases when it worked I was connected to LTE or to my home network. Then I disabled the WiFi and tired again and Siri set the reminder without an issue, so it looks like some of the security settings on the office WiFi prevent Siri from accessing something and therefore "she" coudn't "understand" the context of the request and create the reminder.
Hope this helps somebody!