The most common cause of decreased bloom is an overcrowded clump in need of division. Bloom will have decreased steadily for a few years, probably most clearly in the middle of the clump. Another possibility is insufficient sunlight.
Time is the best remedy for that, but be sure the plant gets adequate nutrients.
Several approaches are possible to extending iris bloom. Consequently, each of their stalks will be in bloom longer. Some have as many as three buds in each socket. More irises fit this category in areas with longer growing seasons, but two that are reliable most places are SDB Baby Blessed and TB Immortality. Some beardless irises exhibit extended or repeat bloom as well, e.g., SIBs Lavender Bounty and Reprise.