The hunkering down in a quiet spot is a wild reaction - deer do it, and wild cattle for example. Once the lamb's stomach is full it goes for a snooze, out of sight of predators. I didn't notice the bawling mums with my Soay particularly, but we have a 15 y.o. ewe this year, with a single lamb, who is forever charging about the place yelling for him, with her fleece streaming behind her. Meanwhile he's either happily snoozing under a hay heck, or off playing with his mates.
If though the lamb is lethargic all the time, and doesn't feed with any enthusiasm, then you need to check it out more closely