Usergroups permissions questions

[theant]

Hello,
I am currently testing your component on a local server.
I'll explain below what I need it for.

I would like to create a download area on the front end where I can make different resources available to the site's various users and user groups. No one should be able to upload or edit anything on the front end – only download.

I have created two user groups belonging to the 'Registered' group:
- Registered
-- ITA
-- EXW

and two categories in JDownloads:
-- ITA
-- EXW

I have then created two downloads with files:
- FileITA (in the ITA category)
- FileEXW (in the EXW category)

Now I would like users in the ITA user group, after logging in, to see, have access to and be able to download only and exclusively the files in the ITA category; they should not see any other downloads. Similarly, users in the EXW user group should see, have access to and download only the files in the EXW category.

In the JDownloads category and download options under 'Access', I can only select Public/Guest/Registered/Special/Super Users – why don't I also see Author/Editor... EXW/ITA? How do I hide a category from other user groups that do not have access to it? And how do I assign a file or download to a non-top-level user group?

In the JDownloads permissions, I've set 'Download' to 'Allowed' for both the EXW and ITA user groups. In the frontend, after logging in as a member of the ITA group, I can see both categories and both downloads, but I still get the message 'You do not have permission to download every file from this page'

There must be something about managing user groups and permissions that I haven't quite got.
Can you help me?
Thanks