Lack of nutrition is my best guess. Just because their bellies are not bloated doesn’t mean it can’t be a dietary problem.
Identify what you are feeding your animals is very important. Not only should you know to make sure it is safe, but also to make sure they are getting all the nutrients they need. I suggest that be your top priority right now.
I saw that you said you cannot grow grass/don’t have grass. However, if you cannot/choose not to feed pellets grass/hay is almost always a must to ensure they are getting the proper nutrients they need. Have you looked into a fodder system? If you do those right you should only have to buy the seeds once, (I assume getting delivered can be a bit of hassle where you are.)
Hi! I put pictures of your plants into my plant ID app, and the one on the right came back as possibly being White Snakeroot, Ageratina altissima, which is highly toxic. Of course those apps aren't always right, especially when it's not a great picture, but I wanted to let you know just in case!
If you have wheat berries, start growing fodder for them since grass is not readily available. You could also grow a rabbit garden. Kale, Swiss Chard, parsley, carrots, alfalfa, etc... (Give them the carrot greens. Carrot pieces sparingly as a treat.)
It sounds like they are not getting near enough nutrition. Old fashioned oats and BOSS can also help supplement but don't give them too much.
I do feed my rabbits pellets, but they also get all of the above.