There are three reasons why certain messages in a thread might be hidden from view.

1. Some of the messages in the topic/thread were posted by people on your Ignore list. To view those messages you need to remove those users from your ignore list.
2. You have turned on the profanity filter and set it to hide messages containing profanity. To view these messages, you need to either turn off the profanity filter or change your settings to "Strike through profanity" instead of hiding it.
3. There are messages in the topic/thread that fall below your chosen ratings filter/minimum rating. If you want to see all of the messages in the topic/thread, set your ratings filter/minimum rating level to a lower setting.