Is she pulling it out with her teeth, or do you just see a bald patch on her dewlap?
If she's pulling it out, she's probably having a false pregnancy. Is she spayed? If not, then my bet is on a false pregnancy.
If you don't see her pulling it out, and it's just a bald spot, she may have a medical problem. Does she use a bowl or a bottle for water? Sometimes repeatedly getting the dewlap wet from drinking out of a bowl can cause a fungal infection.
Check for skin irritation and redness.
--Dawn