Do you know what flies you are feeding? you could try breeding stunted flies if they are still not interested in normal melanogaster. To make stunted flies just add more flies or less culture medium when you make the cultures and you get smaller flies.
I am surprised tho as the two imitator species I keep take the small fruit flies without any problem. If there is still plenty of springtails they'll be getting plenty of those. Worth adding lots of leaves litter if you haven't already as the springs love this and the frogs will love it too.
Hatchling milkweed bugs and pea aphids also good alternatives.