In Stock filter was properly working; at a time I don't know after what; it just disappeared. I checked that the filter is still there, active and valid; but it wasn't showing up. I just deleted the filter and re-created it. Then it started working again. After a couple of days, it disappeared again. Now deleting and re-creating the filter doesn't work. In stock filter never showed up again. Btw yes, I removed the cache for a couple of times.
I have this problem with other filters too. Just disappearing one by one..