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I would think it likely that the lamb has had insufficient colostrum, so I would be trying to get at least 50ml colostrum into it, pref twice that. By tubing if it won't take the bottle. Generally, once the magical golden fluid does its stuff, the lamb will eat with gusto, from ewe if she has some and is willing, or from bottle if that's the only feed on offer.
When you say the lamb did suckle, did you explicitly witness mouth latched onto teat, sucking and swallowing? If so, then it looks like ewe may be short of milk, so you'll need to top up. If you didn't explicitly witness all of those things, then my money is on that lamb did not in fact suckle to effect and did not get milk. (They can look like they're suckling effectively, but are actually on a bit of udder with no teat.)
Have you tried to express milk from the ewe? If yes and none forthcoming, does she feel flabbyish (emptied) or firm (not yet dropped any milk.)
Factors at play could include ewe not yet come into her milk, ewe not actually allowing lamb to latch on and suckle to effect.
If you can get the lamb accepting a top up, then keeping it with the ewe and topping up enough to keep it alive and vigorous but stopping short of feeding it to repletion, is more likely to result in her having a decent supply and meeting the lamb's needs. If there is any suggestion that the ewe is avoidant, then I'd be penning them for feeds and helping the lamb get latched on if need be. Generally, if there is grass and the ewe is attached to the lamb, they'll do better outside if you can deliver feeding support three or four times a day. If any of those things isn't true, then I'd probably pen them until the lamb is feeding successfully. In all cases, topping up from a bottle if needed, sufficient to keep the lamb alive and vigorous but stopping short of feeding it to repletion, so that it is still hungry and keeps trying to feed from the ewe.