I wondered the same thing and have put it down to a timeout issue. If the server has to check your device ID when you connect, there may be a long execution time and the connection time's out.
An IPTV Reset clears what the server has stored for your Device ID, so when you next log in, it just has to save the new info., the connection is fast, and you have success.
Using Kodi 15 versions, if you have not manually edited the PVR Stalker "Settings.XML" file and changed your timeout to around 60 seconds, your connection timeout will only be ~15 seconds.
Using KODI 16 (actually 16.1 RC1 (Video bug Fix)), lets you set timout to 120 seconds, and I haven't had a problem since(Fingers X'd).
You can also just leave KODI running without being on a channel, and this will keep you logged in, which also solves this problem.
/kens