I get it to work with Siren on both my iPad and iPhone. You need to give it about a half a second between presses. Also it usually takes about a second to hear the beeps and see the response, so don't rush it.
I think this issue is really more about the radio than it is about the app. I've only had my 536 for a week, so I'll admit I'm far from having it all figured out. But what seems to happen with the 536 when I hit channel hold twice is the radio first looks elsewhere for activity within that same system it was stuck on. It doesn't move on to the next system first, or the next favorites list, it looks within that same system that has the boring broadcast that I want to skip past. If it finds another active channel within that system it will stop there. And that's fine I guess. But if there is no other active channel the radio comes right back to the boring channel. So I think this is my problem with the Siren app. Sometimes it moves along after a double press of channel hold, but more often than not it comes right back to the channel I don't want at the moment. The audio delay with Siren makes it harder to get a sense of what's really happening, but I believe that's the problem. On the radio I usually just twist the VFO knob when I want it to resume scanning. That seems to be more effective, and I'm not bothered by the change in scan direction. But the app doesn't have that option obviously. Kinda frustrating.
One feature I really hope Uniden can add to Siren is the ability to re-boot the radio from the app. The ability to clear all temporary avoids easily would be a huge help in operating the radio remotely.
G