If you go on the web ui and look where the sort option is, there's a little ?. Click it and it will explain the sorting options.
Many instances use Active as the default, which is based on latest comment. However, it's also based on the score, so if you are spamming or people don't like your post you'll get down voted (or no one will upvote) and then it won't hit the front page even when sorted by Active.