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=178608.0