frankly we used ;human nipples of various sorts from all the bottles that people would forget at the zoo; i had a plethora of nipples and bottles and would try varius ones until i found one that worked;
like someone said: put baby in lap (sit on floor much easier this way), with their knees folded up underthem (yes they kneel down and stretch neck up to nurse), place hand under bottle, with bottoms of your hand and fingers gripping kid's chin on eeither side (like bottle feeding a baby , we were taught to always have our fingers (pinky ) at least touching baby's cheek to stimulate nursing) and scratch around his tail... wagging tail means suckling instinct kicks in. without that, he wont suckle...
we also used to take a small amount of honey and put on nipple, or squirt warm milk on nose and lips and while it was licking, shove the npple in and if u spill some in mmouth, also massage throat, it will swallow, like the taste, and look for more.
if that doesnt work, try pan feeding (teaching baby to suck /drink from flat shallow bowl, but measure the amount before and after, adn away from other animals that would like to empty the dish.
if that doesnt work, see if anyone in your area has frozen milk (i used to freeze colustrum and milk for those in our area we could use eachother's milk and it would have the same local antigens)... or milk someone else's goat/cow/sheep -
at about five weeks u can introduce grains , a few at a time, to stimulate the rumen, and i think in the states there is also rumen starter that u can give also... good luck...
also make sure the kids are warm, a cold kid wont eat. i have kept kids in bedroom /in bed!! with us the first week, and offer bottle every time it would cry (like a real baby), so even if it didnt nurse well, it got to eat small amounts all day long (rather like with mom)... kids like physical contact, nuzzling, snuffling, stimulate their little behinds, like mom would do... all this encourages feeding responses. it workes with nubian ibex orphans, gazelle orphans, deer orphans and kids and lambs, not to mention various other mammals