Are you sure you're looking at their "tame" value (shown as a number on their item icon like weapon durability) and not their hunger or water or relationship value? (shown in their NPC status).
Tame value can't go up on their own outside cages. They only go up when you put a captured NPC (animals and monsters included) inside a cage/fence and put the appropriate food or favorite item on the final slot of the cage/fence. No tables are needed; tables are for putting food as decoration to provide buffs when you're raided.
Even then you need to wait. Basically every time you reach a new "time slice" on your clock, caged NPCs will consume one unit of food or item (if it's listed in their favorite list) and their tame value will go up accordingly. You get a bonus to this depending on your Persuasion skill.
