At this point, I think you need to set up a few controls to isolate the troublesome variable.
It may not be healthy for your frogs, but I think you need to set up a few test cultures using a variety of recipes. For one, you might try placing some of these flies in a container with a slice of apple and see how long they live. It may prove nothing, but it might make you feel like a success for a moment!
Personally, I like to use potato flakes as a base, though I sometimes use corn flour "masa" or a mix of both.
The simple recipe for success is raising flies is potato flakes and water. If you add a spoonful of larvae into the media from a pre-existing culture, as another member suggested, you will be off to a good start as this deters mold from forming through their "turning" of the media as they eat through it. Adding more than 30 flies is recommended, though more is better.
When my cultures are slow to start, I often add a small slice of apple. This gets the flies excited in lieu of yeast, which provides the "rotting fruit" cue that attracts flies. D. melanogaster are fine with yeast, but I've had less success using it with D. hydei than the "wait and fruit" method. D. hydei appear to hold on to their eggs if the media is less than perfect sometimes.
I do often mix in a bit of vinegar, or even red wine...just a teaspoon or so (melanogaster only). If I eat some applesauce or drink a bottle of juice, the unwashed vessel will often become a new culture of flies!
None of this is a perfect method in preventing mold. I don't worry about spooning out a bit of mold if it appears and usually drop a bit of fruit in at this time if the culture seems slow to start.
Raising fruit flies is a science for many people with their old or new recipes, but it sounds like you might benefit from setting up a few cultures and playing around with them. Of course, I will mention again that what I raise flies for is mantises, young tarantulas and other spiders and various other predatory pet bugs. Your feeders are intended for frogs with apparent nutrition requirements that extend beyond what nature's fruit flies probably provide. I must imagine that the frogs eat a real variety of bugs in nature and glean the necessary nutrients from these bugs that feed on various foods. Lastly, if you take anything from what I've written, test out a few different recipes with a lot of cultures. Drop in a small bit of fruit if you don't see larval production after 7 days. It works well enough for me.