Could be egg bound. Is there another Angel in the tank (male or female?). If not, then she won't get the encouragement she needs to lay the eggs.
Could be constipation. Has she eaten in a while?
You could put her in a hospital tank with some Epsom salt.
Start with the tank water she is in, and test the GH. Then add enough Epsom salt to raise the GH about 2-3 degrees.
Could be parasites, but this usually comes on more slowly. Try the Epsom salt first, and if that does not work then you could add worming medicine while she is still in the hospital tank.
If it works, I would treat the main tank, too. If one fish has parasites, the others have all been exposed to them, and it is easier to treat when the problem is small.