If the calf won't suck your finger sometimes it helps to give them a little chip off of a cow asprin bolus, that will really perk them up if you have any available.... Some calves are moist or windy the first couple of days and have trouble sucking, we give them antibiotic if the aspirin doesn't straighten them out. Most calves will at least suck a finger even if they are not getting the hang of the bottle. We have tubed some calves that have very low suck reflex. If you don't know how to do this, you should get your vet to do it the tubing if you are worried the little baby is getting too hollow.
If the calf is just being stubborn, you will just keep working at it. IMO Milk replacer MUST be at perfect temperature or they will reject it, too cold or two hot they play "goldilocks". When I mix milk replacer I add the hot water first and melt the milk replacer, then I add cold to perfect temperature and pour into the bottle. Mixing in the bottle can cause clumping and some say "yuck". Make sure your milk replacer is strong enough too. I feed a whole cup full on new baby calves to a large two quart bottle. Small calves get less volume, (a quart), but quickly they want 2 quarts at a feeding. Same milk powder amount.. Your milk replacer should come with a cup, it is bigger than a kitchen cup. One cup per baby. Reduce as they begin creep feeding. Make sure your baby is pooping and not bound up, or loose for that matter...
Brown swiss calves are the most difficult to break to drink from a bucket and can be super stubborn about the bottle at times, just all bumping and no drinking. I don't think you said what breed the baby is.
If the calf seems winded after drinking don't ignore that, they will get weak and can get pneumonia. Some get moist sounding in the first week if the weather is crappy or they were born fast and not enough time in the canal to squeeze their lungs out. I feed lots of baby calves, we have a dairy and I am feeding milk replacer to 13 of them right now. We feed two times a day, that helps them be hungry when you come with the bottle and they train fast to a bucket too. More often is ok too, but dribble feeding them can wear you out. Good luck