Often, I hear complaints from members that important threads "disappear" as other posts are made on the Forum.
This problem is primarily a function of individuals viewing the Forum using the "Recent Posts" function, rather than navigating off of the Home page or "Recent Threads" function.
If one wishes to follow a thread of particular interest, it is best to use the Forum's bookmark function.
An advantage of bookmarking is that you can receive notifications letting you know when topics you have bookmarked have been updated.
These notifications range from having a notification come up on the forum, allowing you to see at a glance that a topic has been updated, or you can receive emails letting you know when topics have been updated.
You can modify your notification options in your profile settings.
More information on bookmarking is available here.