Because i'm tired of repeating myself, here's a (hopefully) hard to miss guide instead.
SFM will sometimes have problems downloading large files so there are a number of ways to alleviate the problem:

1. Restart the program
Restarting the program will refresh the workshop checker thingo and it will download any missing files. If it doesn't work the first time, try it a few more times.

2. Unsubscribe and resubscribe
Doing this while the program is running and (similar to 1) it will download any missing files, hit cancel when prompted to uninstall unsubscribed addons. Again, if it doesn't work the first time, try it a few more times.

3. Manually download and install
Using the website "steamworkshopdownloader.io" you can manually download the files and install them into your SFM directory. Simply place the Models and Materials folders into "Program Files (x86)\Steam\steamapps\common\SourceFilmmaker\game\workshop"
No longer working as the site has been taken down

4. Locate any broken/corrupt files in SFM's directory, delete them and restart SFM and it should (try to) re-download the missing files