I'm new here but have been fiddling on with video and compression for some while, I hope someone will be able/willing to help me. I've recently started doing some chroma keying in FCPX and that's been going reasonably well, for a very basic setup. I do my keying in FCPX, generate a master as either an Apple ProRes 422 or ProRes 422LT file, the results to this point are fine. I then compress the file using Compressor 4.1.1, I've been producing a .mp4 file which as far as I can see conforms to YT "specifications" such as they are.
The quality of this file also looks fine. If anyone is willing to help me I have uploaded a 30 second clip, the clip is a sample .mp4 file of the type that is being produced from the ProRes422 file by Compressor, the file is about 80MB, it can be downloaded here:
What does the YouTube video look like at 1080?
It's hard to know what's happening without seeing how YouTube is handling it.
Since you're using 50 progressive frames instead of 25 perhaps the bits per pixel is lower than it could be. Try doubling the data rate.