This is either because your file path configuration in your AddForm is different from your configuration in your EditForm. Also, if you're using ResizeVersions you must specify at least one version without a prefix. See step #5 and view the ResizeVersions property which explains this in detail.
Read the first step, Pre-Installation in regards to large files. What's happening is your web server is stopping the file upload from completing because of file size limitations.
You most likely have an error in console. Using developer tools open your inspect panel and click on console. Is there an undefined error? This means you're using an older version of jQueryUI. See Pre-Installation instructions on how to solve this.