Yep - dont get annoyed with them - as it can happen when your tired and pushed for the time for the rest of the flock. But dont push your time with something and leave the rest of the flock.
A tickle to the top of the tail sometimes works, keeping your hand cupped over their noses sometimes works, sometimes they cant create the vacuum needed so fingers down the side of the mouth, the intial 'tip of the bottle sometimes frightenes them as it pours out so squeeze the bottle to get a vacume and when you tilt it your not pouting it into theior face
- gently pull the nipple in and out, if you can see a small amount of sucking and they cant pull out the milk dont hold the bottle straight up and force it into them squeeze and let them swallow.
Its all hit and miss - hope it keeps well.