This could be so many different things. It might not be egg bound at all. It could be internal parasites. They even get tape worms. It could be the first stages of dropsy. Usually a female that has eggs can breach them, and they usually eat the eggs. When they do get eggbound, sometimes you can help them, like what has been suggested. Sometimes it's a badly formed oviposter. It could be internal damage, from many things. Fighting, darting real quick and bumping something, even being shook or banged around during shipping. I only tell you this so you can consider other options. Try putting a flashlight behind the female, and see if you can tell if there is eggs, or does it look like the stomach is swollen. Look straight down on the betta while its in the water. See if the scales along its sides are sticking out any. Like a pinecone. I apoligise if I'm off base here. Maybe you know exactly what is wrong. If not, look for other things than just eggbound.