Read A.S.A.P!

Hows she doing?
She may be stressed, keeping her from birthing. What are the conditions in your tank? What are your reasons for not leaving her in the main tank? If you dont mind the possible guppy population explosion and maybe have another fish that will eat the fry, Id put her back in the main tank. She may or may not birth, but either way her body will take care of itself (if the tank is otherwise ok...again, what are the contitions if the tank? What size, what other fish, water parameters?)

If she is getting thinner then she has had her babiesa nd will have eaten them. The gravid spot will remain even after birthing depending upon the amount of unfertilised egss she is storing for later births. Guppies can have up to 6 birthings from stored sperm. Keep a note of this birth. Guppies give birth around every 28 days. If possible put alot of extra plants in the main tank she and the fry can hide in next time round and dont worry she will be fine.

Im wondering though why not just leave her out of the breeder net altogether?

I have never had good luck with using any kind of birthing unit for guppies (nets, traps, etc.) They get very stressed out when confined in such a small space. If you want to save the babies, might I suggest you get a seperate tank (5-10 gals) as a birthing tank. Put in a cycled sponge filter and a heater(if necessary) and load it up with a ton of live floating or bunch plants (I love najas grass for this). When she starts to get big and plump move her gently (as not to stress her out) to the tank all to herself. Feed her like normal and check the tank often for babies. Once she's thinner or you see babies take her out carefully and put her back in the main tank. This I have found is the best way for both the babies and the female at least for me.

I hope this helps!!