Calls are limited by Twitter and each time you open app it fetches the maximum account info for you that Twitter will allow per 15 minute period. This is stored on your device and we then go off and collect more when synchronising next time. You can either dismiss the message and work with what you've already got, or exit app, wait 20 minutes, then re-open app.
When you close and open again in 15 or 20 mins, the other accounts (if they were missing) will appear. If you follow way too many people (say 5,000 or 10,000) this may take some time. I got the error message quite a lot when I was following 3,000 but don't get it now I've halved that.