FCPX project not uploading to Youtube

I have successfully uploaded many videos my Youtube channels with little or no problems until this last project: it only loads halfway (the upload "circle" fills up halfway and then stops) and then stalls. After a couple of days, I try again. Same problem. My Macbook Pro, late 2012 model, is running Sierra 10.12.6. I would appreciate any ideas on how to solve this. Thanks in advance.