Do you know when she last laid an egg?
Does she feel unusually heavy when you pick her up?
I'm wondering if she might be laying internally rather than egg bound. Laying internally is where eggs released from the ovary, fail to travel into the oviduct and fall into the abdominal cavity instead. It is quite a common problem and can go on for several months. The mass of eggs builds up inside them and fluid builds up around them (ascites) and puts pressure on the heart and lungs and digestive tract. Draining the fluid, if this is the problem will give almost instant relief, but will not solve the underlying problem of internal laying. Some people get hormone transplants to stop their hens from ovulating and this will prevent further build up of egg yolks and fluid in their belly, but it is expensive...... anyway, if you haven't had an egg off her for a while, this might be another possible diagnosis.
Good luck with her.
Barbara
