I just had a septoplasty two weeks ago. My breathing has not improved. Is this normal?

It depends. After nasal septoplasty surgery it is normal to have some swelling for a few weeks and even months. This swelling causes nasal congestion. You should see your surgeon to be certain you do not have a more serious problem such as a hematoma- which is a collection of blood inside the septum that can lead to infection or cartilage damage.
Yes. I tell my patients it takes up to 6 weeks to notice an improvement if there is going to be one. I liken the nasal lining to a nicely waxed car... Your towel slides across a waxed car. When you cut into the lining for septoplasty and turbinate reduction, you "mess up the wax job" and it takes time for that lining to regenerate. If not better in 6 weeks, don't expect any improvement after that.
Septoplasty results. For most patients, 2 weeks is still early after septoplasty to appreciate results. Factors such as expected swelling, increased mucous, crusting, or dissolving sutures all may contribute to this early congestion after septoplasty surgery. Follow your nasal surgeons care instructions to maximize your healing and results. It may take a couple months before you appreciate the improvement.