Ah, darn it all. I had my best success with a regular net positioned right under the filter flow. Then they hatched in the net and I just turned it upside down! Wish I had a spare tank now as I have some fresh eggs.
I think the problem is that you were just recirculating the same water around with the airstone without having fresh water flow over them, like I did with my net positioned directly at the outflow of the filter. I don't have any proof of that; just my best guess.