After struggling for weeks with a large project. about 600 JPG and MOV files plus WMA narration files, that MM refused to create a movie file from I finally got it to work by splitting the project in half then making the MP4 for each and merging them together in another MM project file. The merged project then created the MP4 successfully.
I previously described the problems in http://answers.microsoft.com/en-us/windowslive/forum/moviemaker-files/windows-10-movie-maker-will-not-save-movie-to-mp4/e0a94146-cbab-4305-9c42-d4e6a77957ad
The take away is that MM will not create a movie file, either MP4 or WMV, for projects larger than about 250 - 300 images.
Rather than wait till the end of the movie creation process to let the user know that the process failed and then corrupt the project in the process, MS should either tell the user that some limit has been exceeded or better yet the user should be told at the beginning or during the project build that a limit has been exceeded.
What say ye MS Movie Maker Team? Are you out there?
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