Have you tried differently delivery methods for the hay? I know the common recommendation is try a new type/brand of hay, but for me what did it was (shockingly) the delivery method! My bun’s cage came with the little hay dispenser thing that attaches to the side of his cage, but it could fill it full of hay and he would ignore it. I’d also put his pellets in a dish with hay so he could make a game of finding his pellets while hopefully taking in some hay, but all he did with that was make a mess. Some people have those hanging balls they put hay in, but I couldn’t find one at my pet shop, and oddly my bun also didn’t like it in his litter box. Then one day I took a normal, empty kleenex container (made bunny-safe by taking off the plastic film stuff) and gave it to him. He though it was the best thing ever peaking his head in and out, flipping it on its side, he loves it! So i put some hay at the bottom of it and since then, it’s his favourite thing! Best thing is that even when the box is flipped on its side, since only the little middle area where the kleenex was dispensed is cut out, the hay doesn’t fall out everywhere and the majority of it ends up in him, not around him! So maybe try and find a fun delivery method that he would prefer to it just being there!