How old is the calf? Did it get colostrum within the first 24 hours (12 hours is best)? Did you pull the calf, or did the cow have trouble having him?
If the calf had a difficult birth, that could be what your problem is. Sometimes with a hard birth their head and tongue will be swollen up, and if the calf was deprived of oxygen during the birth, that can cause him to be slow to suck, and you may have to teach him how and where. If his tongue is swollen, that makes it very difficult for him to suck, as he cannot get the tongue wrapped around the teat well enough to suck. With time the swelling will go down.
Since he is sucking a bottle, you should be able to teach him to suck the cow. You will have to restrain her, a headgate would be best, but a halter will also work. Headgate or tie her up somewhere where you can ALSO tie a hind leg back. Unless she is used to being worked with, it is a good idea to tie a hind leg, so she cannot kick you. Get a rope looped around her hind leg (be careful doing this) and pull her leg back a bit, just far enough back that it is not off the ground, but that she cannot bring it forward to kick you. Now you can get the calf up to her and work with him, on the same side as the tied leg. You will want him hungry, as he will not be at all interested otherwise. Maybe give him enough milk to whet his appetite, and so he is looking for more, then get him up to the cow and get the teat in his mouth, stand behind him and push/hold him while you or a helper works to get him sucking.