This one might help you to figure it out:

https://bitcointalk.org/index.php?topic=2818350.0

Furthermore, with the old ranking system, which only depends upon number of activities. In terms of activities, you can reach maximum 14 activites for each two-week period, nothing more even you post hundreds of posts during those two-week period.

New ranking system has added merit scores as one of pre-requisite for ranking up. Please read the topic via the link I give you for more details. Merit system is a hot topic recently in the forum.