Our cat was unsuccessful with her first two litters. All died just a day after giving birth. This time though, she gave birth someplace else and brought home her kittens just last month. We think it's because they're old enough to eat solid food.
I'm proud to say that she's taking great care of them, not letting them out of her sight. The kittens used to only stay at the back of the house. But now that they're more active, they've expanded their playground and reached the front of the house and out in the street. This makes the mother a little apprehensive that she follows and carries them back inside.
She also developed a mother's courage when before most cats in the neighborhood frightened her. She even fights off this little neighbor's dog that keeps going inside our yard. :)