I can't seem to get the potions to work during battle. I have a Revivifying Pylonic in my armory as part of my battle set which should heal 100 HP, but it never actually gets used/gives the effect when I use it in a turn. I left the game for a few years and only just came back, but I swear it wasn't too complicated before.. so I'm not sure if I'm missing something silly, or if it changed the way its used?

Potions have to be equipped directly to your pet. I'm not sure if they can be used via the armory anymore. Remove it from your armory, then click on it in your inventory and equip it to the pet you want to use it with during battle.

Also some challengers have healing disabled. So no matter what item you use, you can't heal during that fight. (Example challenger: Eclipse)