This may not be a Nzb360 issue but I'm hoping someone here may have some input, I've also asked on the Sonarr forum.

It's only happened lately that the artwork doesn't show in Nzb360. At first I suspected the app, I've reset it and configured it from scratch but it's still not appearing. Then I changed Api to see if that forced a fix but no good. Everything lists fine just no artwork. Strangely enough artwork for Upcoming works, just not for series.
Where do I start to debug this?
well if you want to try and take this on yourself you could start with installed a proxy server and intercept the traffic between the app and server. I personally use Burp suite for this. Have it listen on all interfaces, then in the phone's wifi settings go to advanced, enter the ip and port of the proxy server and make sure ip settings are on Auto (not dhcp).

Then reproduce the issue and look in Burp's history to see if it returned an image or not. If not, the issue is with the server. If it is, then the problem lies with NZB360. In that case let us know so we can work from there.
Cheers, I'll give something a go tomorrow evening. I have a decent Mikrotik router, I can packet sniff on that without setting up anything.