Sorry to hear they've all gone. Maybe one or two managed to hide in the main tank before you'd transferred the others into the main tank. My first one was about 3 weeks old when I first saw it so it is possible to miss them for quite a while. Next time she gives birth try leaving her in the main tank if it's now planted up well and see how they fare on their own. Better to lose a few to her natural instinct to eat them and the strongest and fastest will find somewhere to hide - it's natural for them to do so. All may not be lost, but if they are, try not to take it to heart - there's always next time!