I'm new! Fantastic game!
So I'm trying to make sulfur, and I don't understand why this doesn't work

http://i.imgur.com/oeZHJ5a.jpg
The 2 plants on the top side don't work, but the pipes are connected and they're full of petroleum, but the plants don't get filled. What am I doing wrong?
The 3 plants below work fine though.
Thanks!