VC1 is quite problematic codec for Nvidia Shield so Plex tries to start convert the video. So it is all about the used codec itself. If you try to watch the same video with Windows Plex application, it propably plays directly without issues since Windows supports that codec. If you like to watch VC1 titles with Plex in Shield and with portable devices, I recommend you to convert VC1 to h264 with HandBrake for instance. I have done that with my old HD DVD movies which all seems to be VC1 and with some of the older VC1 Blu-ray movies.
Here’s some information about the conversion: Batch converting VC1 to h264 MKV
It’s also possible to try VC1 title with Kodi in Nvidia Shield (direct play always). I have tried and it plays better, but I experienced some hiccups with that kombo as well.
