They can be very sensitive to temperature changes, so coupled with the change in home and losing their mum, a bottle that’s a little cooler or a little warmer than they are used to might put them off.
It’s actually rather easy to burn their mouths if you’ve judged the temperature wrong. My test is to close my eyes and squirt some milk on the back of my hand. If I feel it, it’s the wrong temperature. If it feels warm, it’s too hot. If it feels cold, it’s too cold. Do this test as you are stood next to them, about to feed them, not in the kitchen before you carry the bottles across the blowy yard to the byre...
If they have burned their mouths, it may also cause dark stools as the over-warm milk caused damage as it went through their digestive tract. They usually recover, unless the milk was really hot. But you might need to tempt them to eat for a few days. Sometimes mixing some yoghurt in makes it more palatable, and if they’ve been burned, they may want it a bit cooler than you would normally feed it.