Cubby's birth mom only had 2 kittens. We did have a cat that had them out side then when she trusted us enough she would bring them in. It took her a day to move all 8 kittens into the house but it was well worth the wait. I agree with every one else keep looking. Follow her as much as she will let you. What we did when our cat had kittens outside, she would come in to eat then go back outside we would stand on the back porch to see where she went then after she got far enough away we would start walking and she would lead us right to them with out knowing we were behind her. If she wants them to hide that much you can also try that. Just make sure you keep a close eye on her.
Katie
P.S. Good luck