This is very odd. Maybe post a support incident. It thinks that it uploaded the file, so it probably did. Somehow it may be uploading to a different directory than you expect, or the file is getting deleted... I'm really not sure because this is not a known issue, but we are very interested in reproducing the problem and finding out what might be the cause.
You could also try posting the code for the upload here and I can look at it to see if I can spot an issue. There isn't much to go on with your description other than it doesn't work.