udemy-downloader: [BUG] it does not download the attached resources or subtitles.

Description When you have already downloaded only the video files of the classes, and you run the script again, this time specifying the arguments to download all resources and all subtitles: The script does not download any of the attached resources and downloads only some subtitles from classes (very, very few).

To Reproduce

1. Run the script without downloading attachments or subtitles:

python main.py -c https://www.udemy.com/courses/myawesomecourse -b <Bearer Token>

1. Wait for it to end.
2. Run the script again, this time with the --download-assets --download-captions -l all arguments in order to complete the previous download with the attachments and subtitles, like this: python main.py -c https://www.udemy.com/courses/myawesomecourse -b <Bearer Token> --download-assets --download-captions -l all
3. You will find that upon completion, it downloaded the subtitles for only some classes, and none of the attachments.

Desktop: OS: [Windows 10, Linux Centos 8.5 Stream] Python: [v3.9.1, v3.6.8]