One of the things I have found is that hay quality varies greatly. Even if it is from the same field the quality varies greatly throughout the year.
I consider the first cutting of hay goat quality. It is usually stiff and thick and with the consistency of straw more than hay. My pickier rabbits just wont eat this type of hay. Even some of my big chewers will waste more of this hay than they eat. Goats on the other hand will chew on pretty much anything. They do great with this type of hay so that is why I consider the first cut, all goat hay.
After the first cut the grass will be softer and fresher. Depending on where you are the fields could get as many as 8-10 cuttings. In the midwest US we tend to get 6 or 7. But toward the end of the season the hay starts to get stiff and not as green. My rabbits do not like the late cut either, though horses, cattle and sheep seem to do just fine with it.
If you just have one rabbit you are probably getting your hay from a pet or feed store. Be careful to make sure that the hay is fresh. In a bag hay can "look good" for years but really after 6 months or so the hay starts to lose its freshness. After a year in the bag it is probably stale. Just like you do not like to eat stale food neither does your rabbit. When you first open the bag it should basically smell like fresh cut grass. If it smells off then it is and your rabbit will probably not like it either. If you buy it by the bale you need to check the interior of the bale. The outside will oxidise but the interior of a tightly packed bale can stay fresh for years. I personally try to stay away from anything more than 6 months old in the summer and fall. Though in the spring and winter as long as it is from a good cutting from the most recent year beggars cant be choosers and some hay is better than none.
Only provide a day or 2 of hay in the feeder. When you take the hay out of the pack it starts to get stale faster. So do not give the rabbit too much hay. And after a few days if the hay is not mostly eaten remove it and replace it with fresh. There could be something in that batch that your rabbit is sensing that is turning it off of that hay. Hay is pretty cheap don't torture your rabbit over a quarters worth of grass.
As James said stop feeding your rabbit treat foods as a regular meal. Fresh greens(including carrot tops) are fine. If your rabbit is eating any pellets then it is not starving. Let it hold out as long as it keeps eating some of its food even if its quite a bit less than what it ate before. If it eats nothing at all give it a very small treat food around dusk. If it eats that then it is more likely to eat more of its pellets, doing it around dusk is important because that is one of the rabbits most active feeding times. Early in the morning(at or just before dawn) is another if that fits your schedule better. If it eats something it is more likely to continue eating with what is available to it. If hay and pellets are available then that is what it will eat. But the best thing you can do once you have it eating pellets and hay is not treat it for a week or so. Let your rabbit get used to a regular diet of hay and pellets.
More important than food is water. If your rabbit is drinking water then its belly is still moving. A rabbit can go off food for several days(on rare occasions) but as long as there is water and they drink it they will be fine. If your rabbit stops eating and drinking that is when it is time to get to the vet. So do not worry about starving it unless it gets to 3 or 4 days with no food consumption. Your rabbit is used to getting a diet of junk food, and it is holding out for more. Once it learns that the candy store is closed it will be back on pellets and hay.