Magnolia - half the buds are not opening

My magnolia has beautiful pink blooms, but only on 2 of the 4 large stems of the tree. Literally one half of the tree has unopened buds. The blooms have been open since last week, and this is why I am concerned and not expecting the unopened buds to evolve. This tree has been growing steadily for the past 15 years. It is the first time I am experiencing this partial blooming of buds.

 

Likely some kind of pathogenic problem like lilac blight or honey fungus. Try looking these up.