How much milk are you feeding them? I had calves that didn't want to eat much, but they were getting a 3 qt bottle which is bigger than the usual 2 qt bottle. Many dairies are upping the milk/ milk replacer to 3 qts per feeding to get better growth on the heifers. Many are feeding waste milk, and a few are even feeding milk from the tank instead of milk replacer because the milk prices have dropped so much. And one farmer that is really REALLY exacting, showed me his figures and what milk had to bring for it to be more economical to feed replacer. He also feeds the VERY BEST quality of replacer, and calf feed and runs an AVERAGE of 70,000 on his SCC in his tank. He wins awards every year for lowest SCC. Way below what most dairies that run 150,000 to 250,000.
So back to the calves not wanting to try the feed. Are they getting hay? cut the hay back so they are going for the feed. At that age they will get more bang for the buck, eating nutrient dense grain rather than hay. Dairy breed calves can be tough. Holsteins especially seem to run in the try feed early or never want to try it. My jersey cross calves seem to be way ahead of the holsteins in trying feed.
One other thing I found that works for me is to have a mix of ages so that the older calves will go to the bunk when I put grain in it, then the younger ones think they are missing something.
There is a "nipple feeder" that is for grain. Maybe Nasco catalog has it? Not sure where I have seen it. As a last resort, it could help you. I think that cutting their bottle to once a day, and physically putting the grain in their mouths is the best way to get them to eat it. And cutting back on any hay.
I have 8 calves on 3 cows, 2 months down to 3 weeks. Plus 1 bottle & 2 bucket calves. Every single one is eating some/alot of grain and I never put the first bite in any of their mouths. They started on their own, with the 2 month ones going to the bunk and the littler ones wanting to see what they were missing.
Just got a new nurse cow yesterday, and the bottle baby is trying to go on her, she is very low down as her udder ligament support is not great. One of the two bucket calves will go on her if I guide it with my fingers in its' mouth; the other bucket calf wants no part and I will have to continue the milk in the bucket as she is a sim x holstein/jer and a nice calf that I hope to keep for a future beef cow. But I want her to grow and milk and grain is the way, since she can't figure out going way down low for a teat when it is so easy to just suck up a bucket of milk. I will just have to get a couple more calves that have never been in a bucket to go on the cow.
Good luck with the calves, there is no one way that works for everyone, and not even do things work every time for the same person.