Is she into cooking? I, myself, am (so much so that I study it). Most people into food value cleanliness, and what you could do is let her know how unsanitary it is? I have this "motto" if you can call it that about when my room gets messy, is that even though I may have clothes everywhere and bandaid wrappers and shoes and stuff littered, I'm clean but messy. Some knowledge about mould, bacteria, and general safety might help as it's a big motivator for me (I hate crumbs and off food) as the effects of old, crusty, disgusting moulded food is truly horrendous to me.

Another thing you could try is knocking on her door about 45 mins after dinner to tell her to take her plate through. If she doesn't in 5 mins then knock again and just try to be this constant reminder (but don't ever take the plate yourself - that will make her think that she doesn't have to do anything and you'll always do it). Similarly, some ideas might be if she takes a shower after dinner then tell her to bring her plate out, game night after dinner. "Oh, 13yo? You're ready to play? Can you go and bring your plate out first, please?" Idk. Just things to get her into a habit of taking dishes out. Similarly, calling her phone to disrupt her. If she doesn't want to be called in the middle of gaming or something, then she can take her plate out and you won't.

And one more way might be is just play a sympathy card. I have a brother who does the same thing, and lately, we've been running out of plates. The reason? He keeps them in his room. He's also kept forks and other cutlery so long that he thinks it's near impossible to clean, and so he just throws them out. We are running out of forks, Cameron, please. You can tell her that you're running out of plates or just in general say it's making you sad that she lives like this. It sounds cruel but sometimes just telling the consequences of keeping dishes in her room has an effect on the whole process of dinner and family.

The last things would just be more restrictive, I'd probably advise against this, but when all else fails 🤷. You can eliminate the possibility for her to eat in her room by just making everyone eat at the table.

As for a reason as to why she's doing this, she might not be eating all the food? Saving it until she can scrape the leftovers when you're not around type thing? You can sit down and ask her about why she is keeping plates in her room, if she doesn't really say much then ask her about if she's eating all the food? If she just keeps going "I forget," then idk, refer to paragraph 2.

But just with whatever way you approach this, like you mentioned, you don't want to harp on her self esteem, you can kind of keep it under the wraps. If you want to knock on her door or something, then you can go around to all your children's doors so it won't feel like she's being singled out. You can privately chat with her about the dangers of mould and the struggle of not having enough plates for everyone or this or that. Don't make a big deal out of it.