YouTube's API is apparently broken (or limited) where we can only collect a certain number of subscribers per channel. We have emailed YouTube, posted in forums but they will not tell us when a fix will be made (if ever). So even if you have 1 million subscribers, you can only get a small list of them. The actual number that can be downloaded varies due to the YouTube issue. Sometimes, they will download a couple hundred and sometimes a couple thousand.
On Sep 26th, 2015 we published an update where we now store your subscriber list (at least what we can get of it) in our database. Which means we can give you faster exports and there is less data-gathering each time you run the export. The first time you run it, it will take a while but the next time you run it, it will only need to sync subscribers that it didn't sync last time.
We are trying to think of some creative ways to get a list of all your subscribers if you have more than 3,500. One way would be to tap into the GMail API and look for every email that says "You have a new Subscriber XYZ" and then put that channel into our database. Another option is to sort the "My Subscribers" page on YouTube by "Newest" and each time you go to that page, we pick up your latest 1,000 subscribers and save it to our database. Both of these options aren't ideal or perfect but perhaps better than nothing.
On a related note, we might be able to pull all of the public social information for your subscribers from our server which means you would not need to leave your browser running.
We have gone through several iterations with this tool and there seems to be a roadblock with YouTube each step along the way but we haven't given up yet...