Media Encoder won't begin conversion from .mov to .mp4

I have just finished exporting a film from davinci to prores 422 and am trying to convert it down to a mp4 file to upload for others to review online. But for some reason, when I drag it into media encoder, set the setting, and press the start button, the queue appears to start (the start button is replaced with a pause, and the bottom preview screen appears) but nothing happens past that and it just sits there. When I try to pause it and delete the item, it won't allow and I have to force quit the app. Am I missing some compatibility issue between encoder and davinci or with encoder and mov files?