I have an erratic eater--Piper the broodie--and I second what Judy says. If Elsie is not on meds, seems happy, and holds her weight (even if it's a bit under what you'd like), don't worry about her. Give her some time to eat, and if she doesn't finish it, just pick it up. Don't increase the next meal--she's deciding how much she needs.
And don't switch foods to tempt her. Sometimes a switch is a good idea: Perry was underweight for the first seven years of her life and I tried everything to get her to eat, only to have her gain weight when I switched foods because Minnie had to change. But frequent switches and topping changes seem to make the picky eaters even worse. [she says, 'ing herself because she learned this lesson with Perry, and then got suckered in again by Piper.]
You've been through the wars with Beau and Polli so your sensitivity is perfectly understandable. But try not to assume that Elsie is going to turn into a walking skeleton if you don't jump through all the hoops. You have lots of time to panic if she starts losing weight.