What happens is this:

When anything is added to Limewire it is in red and Limewire scans it. Simple as that. Once the scan is complete, it is added to your library (gets out of red to black) and is available to download.

Many factors can slow the scan down:
The overall speed of your system
The amount of processor % others apps are taking
The amount of RAM you allocate to Limewire
The size of the file in question.

Once Limewire does its initial scan as something is added, it never needs to do such a precise scan again, and will never automatically do so. Just be paitient and wait for the scan to finish. It seems as if you have a large file, so it will most likely take some time.