Melissa,
One contributing factor may be overcrowding. A single iris rhizome will grow into a clump of iris rhizomes over a few years and once your rhizomes reach the edge of your pot they will start growing over each other. This makes it difficult for the roots to reach the soil and may diminish flowering. Several iris vendors offer advice on their websites. This is the link to Schreiners' growing tips and it contains a section on "Why won't my iris bloom?"
https://www.schreinersgardens....
There is an example of what a very overcrowded iris clump looks like here
http://alcatrazgardens.org/blo...
