Your Feature Usability tells you whether you can use user-specific features on Sudomemo, such as adding Stars, subscribing to Creators, viewing mail, and so on. It is determined by whether Sudomemo can automatically obtain your Flipnote Studio ID (FSID).
Here is an example of a Flipnote Studio ID: 5700A920CC72C82E
Sudomemo obtains this by sending a small request to the real Flipnote Hatena, and pulling your FSID out of the response. Sometimes, Hatena's server can't give Sudomemo a response. My best guess is that whenever a new DSi connected to their server, it would add them to a database. At some point, it stopped automatically adding DSis to the database, and connecting would fail. DSis after then would not be registered, and as such, Sudomemo cannot get your Flipnote Studio ID.
Unfortunately, I can't fix this. It's the only secure way of obtaining your Flipnote Studio ID.
Some tips: - Never, ever reset your DSi or reinstall Flipnote Studio. A new Flipnote Studio ID will be generated and you won't be able to use the user features. - 'Reload User' is for when your session times out. If it does, try disconnecting from Sudomemo and reconnecting, then trying again. If that doesn't work, try closing Flipnote Studio and then reopening it, then connect.
Sudomemo can still be used even when your Feature Usability status isn't usable. You can still have Stars added to your Flipnotes, and people can still favorite you.