|Why hasn't my 4 1/2 year-old magnolia tree bloomed?|
Some common causes of poor flowering are insufficient sunlight (these trees prefer full sun but will tolerate partial shade) and bud damage from spring frosts. Some magnolias in the South even bloom in February, and these are especially susceptible. I don't know what kind of magnolia you have, so I can't say whether this is likely in your case. The Southern Magnolia blooms in late spring or summer, so it's not as easily damaged by the cold.
If your tree is planted near a wall of the house where it gets morning sun, that could trigger the tree to heat up and begin growth prematurely. Thus flower buds are more easily killed by the cold. Magnolias are best planted where they are sheltered from morning sun. Low-lying areas at the base of slopes are likely to be especially cold, so these locations should be avoided. Of course, this is a more common problem in the north, but it's one situation to consider when magnolias fail to flower.
Magnolias like deep, fertile, moist soil with good drainage. Mulch the soil around the base of the tree to help keep the soil moist, and water regularly and deeply during dry spells.
I hope this information is helpful. Good luck.