HOW-TO Get old articles

Newsgroup - Properties - General tab
Use this tab to fetch older articles, or all available articles, for the selected newsgroup.
The number on the left of the slider represents the lowest article number available on the server (the low water mark). The number on the right represents the highest available article number from the server that has not been read. You can get older articles by sliding the slider bar to the left as much as desired.

After resetting the limit, the server count will increase and you can then download the older headers.