Have they maxed out their level? Pins can do that, ya know.
Of course they haven't; I'm not an idiot, you know.
I bet you have an article of clothing equipped that stops you from gaining PP.
Okay, maybe I am an idiot.
This was the problem. Neku was wearing a dress (don't ask) stopped us from getting battle PP. What a worthless article of clothing. I really wish there was a way of "unlearn" an item's ability, because I really liked this dress but I can't wear it anymore.
I'd like to believe that I didn't know the dress' ability when I first put it on and then learned it after entering a shop, but chances are I just wasn't paying enough attention when equipping myself on the start of day 1.
That's my biggest complaint with the game--the menu system is overly complicated. I don't like that info on the pins and items are spread out over 3 or 4 pages. It makes it difficult to look at a large number of possibilities and determine which is the best in a given situation.
I wish there was on option to auto-equip the best items depending on a set of basic criteria (e.g. defensive, offensive, increased HP, well-rounded, better items to go my partner, etc.). It would make up for the terrible menus.