"...please note that only some of the waist items will unlock an additional Potion Belt slot, as long as the belt stays equipped. All these belt items are available from different raids and trials in the game. Also, you may not find the Potion Belt effect specified in all of the belt items that releases one slot in the Potion Belt..."
Regards,
Game Master Jeyra,
Sony Online Entertainment - EverQuest
I found out this belt adds a potion slot when I removed it to upgrade to my new belt. The above is the message I received when I inquired, as I thought I had earned the slot through play.