The problem is that the stolen lamb smells of the other ewe, both from being washed by it and by having drunk milk from her (which makes the back end smell wrong). So she thinks she has someone else's lamb in her pen. Take a handful of her expressed milk and rub it over both lambs, especially their heads and tails (this leaves a horrid mess but will encourage her to lick them both, plus they will smell the same and of her. Once her milk goes through the stolen lamb then it will smell of her properly.
Years ago we noticed our neighbour with a canister of 'Pledge' furniture polish in his quad lambing box.

we asked? He used it to spray a little near the mother's nose so she couldn't smell much in case she was going to reject her lamb (seems counterintuitive to me as she must have bonded by then surely), and to spray onto the lambs to keep the fox away, as these lambs were going on the hill.
It occurs to me that you could lessen her sense of smell with that trick in your situation, but the milk on the head has worked fine for us.
Carry on with holding her at each feed as recommended above of course but this should speed things up.