IMO they need more groceries if they are truly at the body score you say. Poor pasture offers little nutrition, just busy work. Grass hay -- that can be less than optimal unless tested and you know how good it is. It should be available 24/7 right now. One cup of alfalfa is not much....especially consider some will eat more, others get nothing.
If the entire herd is low weight, it is a matter of feed, period. If only one or two, those need extra attention. Just saying, while we don't want to spend more, overfeed, etc., you see them being thin -- feed more/better. If your pastures aren't like those of Sheepshape, you will need way more supplementation. Obviously, her pastures are great! Those girls are getting well fed. Tap the picture & get a close up.
I also agree with Bay, get a good mineral mix, especially with pasture you say you have. It isn't supplying all they need. At some point, you may have your pastures improved and need none to little else.
ETA: You say "after weaning" -- but, it take more input from the ewes body resources to make milk than to grow a fetus. Possibly this is what is lowering their weight.

I don't mean to come across as being demeaning because I am not judging you or your methods, just offering my thoughts based on what I read. Heck, I raise dairy goats and if you saw them some would look thin on their topline...they eat huge amounts and "put it in the bucket"

So I know FIRST HAND, what it takes to make milk. One doe will get pasture, 2 pads of hay and about 8-10 # of feed.

I get 1.5 gal of rich milk and kisses.