Looking at Satori's reaction, it's obvious.

The cat's Maximum MP(magic capacity) is hard for us to measure, but... She probably used to possess a lot more magic power than she does now.

But normally it should just replenish on its own through eating and rest and so on, right...?

Indeed.

Seems like that isn't happening for some reason.

For magical creatures, magic is the same as life force. It's a problem if it doesn't replenish naturally.

We can recharge it with potions for now, but we can't keep doing that forever.

Some flaw in the grimoire that became her core, something wrong with her environment... Whatever it is, the faster we find the cause, the better for the cat.