Yesterday the no beside my username which denotes the no of postings was not ...

That number is added to when you respond to posts! The posting of your discussions allow for added earnings if people give you a plus for the discussion and then participate earning their number of posts. The more you reply as in a discussions also allows for greater earnings. Hope this helps you to understand!!

that happen to me before, what causes it i'm not sure but it will get better in time.

You only get ranking points when you are replying to someone else's discussions. Your number will not go up when you post your own discussion or are commenting on those that responded in your discussions. That does not mean that it doesn't count toward you overall earnings though. It does, so keep posting comments in your own discussions as well as replying to others discussions. It all works out in the end to earn more money for you.