Try a lamb nipple.
I have also made a bigger hole in the end of the nipple, it seems that it runs down their throut and they have to swallow...which in turn seems to trigger the sucking reflex.
When the calf is a newbie I have found that if I rub the calf and physically stimulate him the calf seems to relax more, and gets more in the mood, not so worried about the stranger messing with him.
One more thing is I HATE tubing a calf, It is an absolute last resort. I have found that the calves seem to fight feeding when they have a tube run down their throut. I had a dumb one out of a hfr and it took over 2 hours to get him to take the bottle. Never used to have patience for this, but, it is worth my time to take the time at the start. AND I found that women seem to have more of a touch, they don't fight the calf, the calf doesn't get mad.
Good luck!