Microsoft has released a list of known issues affecting the JTP (January Technical Preview) as it currently is, specifically with Cortana. Though Cortana will be fully functional in many ways, a number of problems have been encountered with setting reminders.
Specifically, once a reminder is set, it is impossible to change. In addition to this, if Cortana is turned off and on again, then reminders will not work and when a reminder is completed it will not show up in Cortana's notebook. Also, when setting complex reminders, such as when adding a location, the message "Sorry, I couldn't create that. Please try again later" appears, though this is not always the case.
As ever, these issues will be fixed in later builds, Microsoft have released this information to show that they are aware of the situation. The January Technical Preview is due to be released soon.
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