She could be stressed. Did you separate her or leave her in the main tank. At one time, I tried using the plastic or net separators and have since stopped because I found that they really stressed out the female. In my experience, the females will hold for a while if they feel stressed so as not to release the fry into a situation that she doesn't feel safe. Could be this, might not be. If she is separated, I would put her back in the tank and take your chances with the fry or put her in another tank.