I had a simple setup with an oil drill heading towards oil storage heading towards a cracker. It wasn't working, no matter what I tried: adding a 20-skill colonist to troubleshoot, adding more power, connecting pipes all around... until I placed down a chemfuel storage tank. After I did that, it started working exactly as intended.

So, TL;DR is try having somewhere for the cracker's product (chemfuel) to go. In my case, it was a chemfuel storage. I hope this helps.