I'm quite surprised actually. Usually after they've been topped up for a couple of days indoors while Mum got sorted, they're running to you shouting (whether they see the bottle or not
), and that transfers to the field without problem, especially if you teach them a "bottle's here" call.
We sometimes use a pen-within-a-pen setup for wary mums with littles that need something doing. Big outer pen, smaller pen (eg just a hurdle across the corner) well inside, where the lambs can be held while getting sorted. Bucket of something nice, or a scatter of something nice, for the ewe. Get them all in the big pen, then while mum is eating take a lamb into the small pen, sort it, swap for 2nd lamb, sort that, bit more treats for mum, put 'em all out. Do not catch or handle the ewe, so she builds confidence that she won't be messed with, it's all about getting a feed. Try to not have lambs screaming for mum, and if they need catching, try to do it swiftly and quietly so there isn't a lot of panic and adrenaline for the ewe to avoid repeating. (Eg., use your body and your crook to quietly steer the lambs behind the hurdle, then swing it shut, rather than chasing them about the larger pen trying to catch them round the neck with your crook, or by the leg.)