Sorry to hear about these issues, so far your the only person who has reported these to us.
Can you try reinstalling both your NOW TV App and also your Chromecast app's and then try again.
Also make sure that your software is fully upto date on your devices.
I can confirm that I'm seeing the exact same behaviour, probably for about a week or two now.
* When pausing playback on my Chromecast via Android, the video does pause correctly but it seems as though the app doesn't realise that this has been done
* As a result, instead of switching to a "play" option in the app, it continues to show a "pause" button (even though the video is already paused)
* Pressing "pause" for a second time causes the app to begin showing a spinning icon, as though it's lost its connection to NowTV or to the Chromecast; meanwhile, there's no change to the video, which remains paused
Generally the only solution I've found is to stop casting (which resets the Chromecast to the standard screen), reconnect and start playback again. (although it does thankfully remember where it left off!) 🙂
I have noticed one other quirk. If I use a second device (e.g. my other half's mobile), connected to the same NowTV account *and* connected to the same Chromecast in the NowTV app, then I'm able to pause playback with one device and then resume playback through the other, provided that I do so consistently (i.e. always pause from device 1 and always resume from device 2). I suspect this is related to the second of the bullet points above: the app can tell the Chromecast to pause / play but it doesn't update its own record of what state it's in, so if one device thinks it's paused and the other thinks it's playing then it behaves as described.... Clearly that's not a solution to the issue, but it could be an important symptom. 🙂
Also FWIW I'm using NowTV app version "5.6_arm" on Android 6.0.1 (Marshmallow) and I have a 1st generation Chromecast.
So our teams have identified an issue with the Android App and the way it handles the Chromecast.
It is getting looked into by our teams and Google.
The issues only seam to happen if you pause something for more than 20 min's.
Once I hear more I will update you.