You do not indicate the maturity of your magnolia. Magnolias do grow in San Antonio, Texas, but need a period of about five years of acclimation to the alkaline soil prior to taking off and doing well. It may be that your tree isn't happy with its soil or location.

There are several things that you can do to improve that. Ensure that there is no grass (or anything else) growing under the tree. Spread an ample application of an iron product such as Ironite or Sulfa-soil around the drip zone of the tree. At the same time apply some magnesium sulfate (Epsom salts). Then apply a 1-inch layer of a good compost followed by a 3- to 4-inch layer of a good organic mulch in a donut fashion under the tree. This should extend at least 6 feet from the trunk. These cultural practices should improve the health and color of your tree.