Have you tried Hills A/D , its highly palatable, high calorie food, that could help. RC also do a powdered convalescence diet that you mix into a milk like liquid. That also is very high calorie. You could try adding those in along side what you are feeding her already. Other things I give my queens are goats yogurt with a little honey and scrambled egg made with either evap or goats yogurt and lashings of butter. Will she eat raw? Beef or lamb mince also helps add substance.

You have to be careful though that if you make things too rich then you upset her tum. But one or two meals of rich stuff along side her normal diet should be OK.