Hi I have been tearing my hair out but I finally figured out why my posters are not always downloading.
A DNS request to thetvdb.com will normally return an IPv6 address as the default instead of IPv4.
As a result when sonarr tries to download the artwork it is trying to make an IPv6 connection.
Most people with modern equipment and operating systems probably have a functioning IPv6
local network. But on the wider Internet, implementation is still spotty. So downloading with IPv6
may be impossible for you. I am curious how ANY of the artwork has loaded.
It would be good if sonarr would try both IPv4 and IPv6 or some fallback scheme. Alternatively
perhaps there could be an option to disable IPv6.
Anyway for anyone having the same problem there is a very simple solution. Add an entry to
C:\Windows\System32\drivers\etc\hosts for thetvdb.com. Use ping -4 to get the current value.
This value will have priority over the DNS result and force an IPv4 connection.
I hope this information is useful to other people and perhaps also the developers.