Certified GKH Gardening Expert
Magnolia trees do not have male and female trees (though some other kinds of trees do). Both the male and female parts of the flower are contained on the same blossom.
If I had to guess, I would bet your magnolia is planted in the middle of a lawn that gets a lot of attention, especially in regards to fertilizer. Lawn fertilizer is high in nitrogen, which makes for lots of green growth but few flowers. If there is enough of it, it can actually stop a plant from blooming.
Try adding some bone meal (or other phosphorous heavy fertilizer) around the tree at least a few feet from the trunk. This will help offset the nitrogen. Also, if possible, try not to use lawn fertilize under the magnolia tree.