Is the tree getting bald? Or still thick in the canopy? Like many evergreen trees, the oldest leaves tend to fall off in the spring, and it is getting old enough to shed an alarming amount, especially after a cold winter, but the younger leaves will remain.
If it is going bald, look for damage to the roots from digging, new pavement, or change of grade. The first two can often be treated with a light pruning and a root stimulator, if not too severe. The last means reversing the grade change over the entire root area, at least from the trunk to the drip line (the boundary under the outermost edge of the tree canopy). Magnolias are very sensitive to grade changes.
Seedlings have been known to take up to 20 years to bloom, but grafted, named varieties are normally much faster. Unless it is a named variety, I wouldn't be alarmed by failure to bloom.

Many thanks,Tuqbrethil, - a great help