You can buy ruman stimulant powder made by Provita and give this to help stimulate her rumination/digestion we get this from the vets. Also as said above and even better is rumen fluid from a doner sheep, we have managed to 'steal' it from other sheep while they are cudding and this is easier than trying to tube it, you need to put a stick like a gag in the doner sheep's mouth so they chomp on it and then scrape is off the stick, best if you have a fairly tame sheep that will keep cudding on the sheep and not panic/run away!
B12 /Intravit can be given to help stimulate appetite.
We used to give mulitvit injections to any sick sheep as advised by VSS but unfortunately it can no longer be given to food producing animals and is now only licenced for horses not intended for human consumption so you would not be able to give this to your ewe. Would be very interested if anyone knows of any alternatives to this, other than this a vitamin drench might be an idea.
Keep going with the energy drenches /yogurt etc - if you can get hold of live sheeps or goats milk yogurt this will be much better than cows milk yogurt, have also heard that guiness is good for sick sheep! Also worth picking tasty hedgerow herbs, like cow parsley, to try and tempt her with. Why did your vet disapprove of ivy, we also offer this too to any sick sheep?