There are a few things that cause you to not gain Post Count:
1) The Thread you posted in was removed or is no longer accessible. This causes your post count to drop rather than to be "on hold".
2) Posting in the Off-Topic Forums does not add up to your post count.
3) Posting right after a different post of yourself adds that post to your previous post rather than making a new one. This also causes your post count to remain unchanged.
I hope this is of some use for you.
Best of luck, and if you happen to come across something that really seems bugged, feel free to post about it here or make a new Thread about it in these Boards (If its a completely different subject).