Woohoo! I took your advice Joan and I didn't cut the worms - this time Stella ate 3 big worms (one of them was the fattest one I've ever seen). I was getting really worried b/c she was starting to look pretty thin - but it looks like her appetite is back.
Martini, on the other hand is having trouble swallowing the worms, it seems b/c they are too long - I managed to get her to eat one. The first one she spat out, so I gave it to Stella, and then Martini came to the front of the tank looking so disappointed, so I found her another one and she ate it. I tried to give her another but she tried once and couldn't swallow it so she gave up, so I gave that to Stella (3 worms is the biggest meal Stella has ever eaten and she's looking pretty pround of herself!
From now on it's just compost worms, and maybe a cricket once a week!
Joan, you are seriously a well of information! thank-you, you've solved my problem!