It's either the topic you replied to got deleted for some reason(probably due to the post being nonesense/redundant), or your reply itself got deleted. You can easily prevent the latter by simply only replying with replies that actually contribute to the topic.
Also, 30 postcount =/= 30 activity. You can only gain a maximum of 1 activity a day.
For more information:
https://bitcointalk.org/index.php?topic=178608.0Thanks for the information. It seems like I have exceeded the maximum post count for 14 days. However, how long it take to upgrade my rank?
Have to wait at least 60 days, and I see the amount of your activity will soon take you up the ranks. only 3 constructive posts are required