Hi Lee, my guess is simply that the upload is not completing, because either:
i) the network connection is not reliable
ii) the file size is too great.
The uploadable file size in Poodll 2 can be no greater than the course or activity's max upload size. If tiny recordings always upload ok, but longer ones do not, then look to update your course max upload size.
https://docs.moodle.org/27/en/File_upload_size
Justin