Siri can not activate some alarms on iPhone 6S Plus with iOS 9.2 When I use the "Hey Siri" feature to enable an alarm it usually works. But recently I've encountered a problem. The conversation with Siri would go like this:

&lt;me&gt;   Hey Siri, wake me up at 7 20.
&lt;Siri&gt; Your, 7:20 alarm is on.
&lt;Siri&gt; (short pause)
&lt;Siri&gt; Sorry Jozef, there has been a problem setting up your alarm.


And indeed the alarm is not turned on for 7:20. It would work for other times, ex. 7:10 (regardless whether an alarm at desired date exists or not).

Is there any special case when Siri can not activate a specific alarm?

This is not a problem with Internet connection (I have one) or understanding of what I said (the text is interpreted correctly and Siri actually seems to have turned the alarm for a short moment).
Be explicit when you ask for an alarm:


  Hey Siri, wake me up at 7:20am."


Also, what version of iOS are you using? There were some issues with alarms not waking people up in iOS 9.1. If you are running 9.1, update to the latest version, 9.2.1, and see if the issue persists.