Playback Problems

I am fairly new at this. I have a QNAP TS-251 connected by ethernet to my wireless router. Plex Media Server is loaded on the QNAP. On the QNAP I have several television shows in mkv format at either 720 or 1080p. My Windows 10 PC is connected wirelessly to the router, and I have Plex installed on it. Most often I cannot watch the shows on my computer as they simply freeze. I also have Plex installed on a new Apple TV, also connected wirelessly to my router. I can successfully watch the same shows on the Apple TV that I cannot watch on my PC. I expanded the memory on my QNAP and it helped a little, but I still have a problem. I just do not know what the problem really is. Can anyone point me in the right direction?