Society Finch wont fly.

Question
I just purchased a pair of Society Finches, and I noticed the one will not, or can't fly. He/she hops to the perches and grabs the side of the cage to climb up. And when he/she misses a perch, they fall to the bottom of the cage. He/she seems to be eating, drinking, singing and interacting with the other finch with no problem. What could be wrong? Recently, one of them laid an egg, which fell to the bottom of the cage and broke. I am assuming the one who won't or cant fly is the female, and she couldn't get up to the nest i installed to lay the egg. I recently put on a stair/perch way for him/her to get around the nest with ease. I just hope this is nothing serious like a disease. Thank you for your time.

Answer
It's hard to say. Try holding the society in your hand and see if she then tries to fly. If she still doesn't, it could be a problem or it could just be an old injury that takes away her ability to fly (i.e. a broken wing).
If the society doesn't fly even when being held in an open hand, compare the wings to the other society, do both birds have their wings firmly tucked in or do hers hang down? Are all the feathers in place or do a couple go out in odd directions? When the wings are spread (do this gently) do both societies have identical shapes? Is she overweight?
You can tell the weight of a bird by buying a kitchen scale, putting a box on it, taring it with the box, and then sticking the bird inside the box. A healthy society should be between 16 & 17 grams.
If the wings hang loose, the bird is ill and a vet may be in order. Put a heat lamp (like those found in the reptile section of the pet store) and keep it between 29-31C (28-30C for healthy birds). Make sure the food has all the vitamins the bird needs. If the bird doesn't start showing signs of improving within a 10 days, take her to the vet.
If the feathers go in odd directions, watch to see her groom herself. If she is doing it properly, then the feathers have grown that way and make flight difficult. This will never change so you will have to put lots of perches in for the rest of her life.
If her wings are different from her mate's then she was injured at some point in her life and probably will never fly. Again, more perches and she'll be fine.
If the bird is overweight, taking a couple perches out will force little tubby there to use her wings and thus lose weight.