MKV is not an encoding but a "container." Bad advice. Try encoding then differently. I can't check each set that folk ask about but what strange and misguided advice you get at times.
Try HANDBRAKE and a few of its profiles if you don't want to search what encodind is supported.
I have a 9 month old Panasonic Viera Plasma HDTV. I also have network attached storage (NAS) that is DLNA compliant. The problem I am having is not all the media that is on the drive shows up on the TV.
I started with 61 video files on the NAS (52 mp4 and 9 m4v). None of the m4v files showed up (some of the mp4's didn't show up initially but did later...). Panasonic told me that m4v files were not supported and suggested converting them to mkv. So, I did that and then found that the m4v files started showing up (I could see duplicates on the TV). So, I moved the MKV files to their own directory but only 3 show up.
Anyone have any idea why the DNLA service on this TV is so flakey? I can see the files from any other computer (mac, Windows 7, smartphone) and all the files show up correctly. Only the TV is messed up.