admin cannot find file

in the admin process queue it has not converted and says cannot find video file .
but if i look in on the server it is there have looked into this a lot today and i have found out that if you upload a video and it says error cannot find the file in the admin process queue list on that file
but if you upload the same file again it works ok as the file is already on the server it is as if it is looking for the file before it is on the server

can you please help as i am pulling my hair out on this

thanks richard