Hello Arakanic,
VFBPro relies on the settings for your actual Wordpress and server for the upload sizes and manner. I suspect that something serverside or website side is preventing uploads in some cases due to one of the following:
1. File size restrictions
2. Server file size restrictions
3. Timeouts (server taking to long to upload the file)
4. File type not accepted by Wordpress Installation or by server
5. Running an outdated version of php on server
Hope this helps,
Joseph
VFBPro Support