Can't create mpeg4 tags

I have a number of mpeg4 files that were produced by Nero. In the past I was able to tag these files with a genre and I'm not sure which program was used to do this. When viewed with mp3tag many of the files report as having an mp4 tag and the correct information is displayed, the other files have tags (since they are correctly reported in the right genre when viewed over an UPNP interface from a WHS system) but they can't be edited by mp3tag. I even tried to delete the tag and that did not allow mp3tag to work (but the tags were deleted since they no longer show up with the correct genre in the video player). Any clues as to what's up?