Their teeth grow really quick so can rapidly become an issue. one of my house buns has broken bottom teeth so her top ones don't grind down and I have to get them clipped at the vets. when they are overgrown she struggles to pick up food.
You can check the front teeth yourself if you get them on their backs so they 'trance out'.
I think naturally rabbuts are quite slim, we are just accustomed to seeing them plump!
we've got 3 outdoor dwarf bunnies. they aren't fat.
between them they have a bowl of muesli, a bowl of nuggets, fresh hay every day (from a farm), and a couple of handfuls of fresh veg every day, as well as a large bottle of fresh water every day.
you don't say the size of your bunnies, but I think maybe you need to give them a bit more dried food than you already are doing.
they are designed to graze all day, so weigh your bunny, then try upping the amount of pellets and weigh again in a couple of weeks. if she hasn't gained anything in this time then I think you need to take her to the vets. they are normally pretty good at self regulating their food and it sounds as though they are getting plenty of exercise and aren't bored.
Vet visit may well be in order to look at the rabbits teeth if they are losing condition.
What are their droppings like? Rabbits do the pooh eating like guinea-pigs do don't they? I rarely find soft pooh in the cage (GP2 has usually got a pooh stuffed away in his bum for later- little swine) so I know the solid ,long droppings are the second passing. They need the nutrients from these first cycle of pooh.
We have two indoor house rabbits, a neutered boy from a friend and a neutered doe from a local reputable rescue. They were both fat when we got them (the rescue said the doe still needed to lose weight) and the boy is looking great now, although I wouldn't want him to lose more weight, but the doe is getting a bit on the thin side IMO.
They get ad lib hay, usually two or three different types and then about 3tbsp of pellets a day between them, either in a treat ball or thrown over the floor. They also get dried 'forage' mixed into their hay a few times a week and veg with some fruit 3-5 times a week. (one of the buns can't tolerate more veg than this so far) They also get the odd 'yoghurt' type rabbit treat that you buy in pet shops, but it's very few as I know they aren't really good for them. They haven't got a cage, but have free range of my sons bedroom with plenty of toys, tunnels etc, they're very active. They do get shut in a large pen in there at night.
I have increased their pellets by about 50% and given a bit more fruit/carrots, is there anything else I can do? I was wondering if readi grass would help with weight? I am also wondering if a vet visit is in order or maybe worming? I'm happy to take them to the vet, but don't want to stress them out unncessarily.