Guppy breeding question?

My guppy STILL hasn't had fry. I bought it a month ago and the women at the pet shop told me she didn't know when it became pregnant but she guessed it would give birth within a week. 4 weeks later. nothing. she eats, doesn't normally hide, square ish but has been for a week now, you can see the little tiny specs at the gravid spot which is very dark but again has been since I bought her, water quality is fine, ammonia 0 nitrite 0 nitrate 15. the tank is 28 degrees C, there are no males but she was with males when I bought her. I have neon tetras (15) platies (2) black widows (3) the tank is 165l

Why is there no fry yet?