Why can't I recover any more health when I try to use a recovery item?

When I'm questing in a dungeon as a character with low HP (Charme) I find myself having to use many recovery items. Unfortunately, after what seems to be a set amount of recovery items used, I'm trying to use a chocolate bar to recover and can't because the game tells me I can't recover anymore. What's going on here? Am I only allowed to use a certain amount of recovery items in a single dungeon run? Why can't I recover?

"Can't recover any further" is generally a message that shows up when you try to eat foodstuffs but the corresponding gauge is full. I can't say I've seen food stop working, and after feeding Nagi 35 foodstuffs (consisting of 23 Apples and 12 Oranges, I did not run into any issue of eating to the point that recovery items flat out cease to function.

Is it possible you're attempting to eat a sweets item (which heals SP, not HP), and thus confusing the inability to recover from your full SP as an inability to eat anymore food?

If you're ever unsure of what an item heals (say, you don't know that Taiyaki is a sweet pastry), then you can always examine an item with Button 3 (default C) to look at what the item does. It will tell you whether it heals HP or SP or both, as well as how much it will heal.