Mcebuddy uses the h.264 codec by default for its mp4 profile. However each converter is a little different and is possible that some of the options used in the conversion may lead to a file with an unsupported feature. For eg some devices do not support
b frames. The mp4 profiles have b frames but the m4v profiles do not since many apple devices do not support b frames.
Try this, use the m4v fast profile it's. pretty basic profile and see if that file works on your device (you can rename the file to mp4 after the conversion if your device does not support m4v extension. There is no difference)