Media Encoder - Could not be imported. Could not read from source (error)

Hi,

I'm trying to import a CCTV file (.264) so that I can export as a MPEG4 file. I have done this with various other files from the same location without any issues at all but for some reason this file doesn not want to transfer. Even when I move the file from it's location, it still shows the same error. Can anyone help?