"Could that be the reason why some posts do worse?"

I would say, probably not. There are simply so many variables. The most likely explanations for most people, when they ask this, is they are:

1. Not networking
2. Not posting high enough quality
3. Haven't been persistent enough
4. Have unrealistically high expectations

"How are they doing this? If there is a secret to success (besides all the other recommendations I'm already following) please share =)"

Ask, and ye shall receive.

https://steemit.com/steemit/@lexiconical/steemit-like-life-is-a-popularity-contest-embrace-it-or-devote-your-efforts-to-other-pursuits-your-problem-is-not-the-reward