Some years ago I planted some iris bulbs (not rhizomes) and they subsequently flowered for three seasons.?

What do you use as a fertiliser? If you constantly use too much nitrogen that will upset the nutrient balance and you will get plenty of nice dark green growth but no flowers, try using a complete plant food, it sounds like there might be a phosphorus defiency or too much nitrogen in your fertiliser, i cant be sure.

Iris need to be dug and and divided every three years or so. They multiply and then get crowded, making it difficult for them to get the needed nutrients from the soil.

Do not transplant the iris if it is in bloom. Most gardening websites suggest you divide and transplant in the late summer or early autumn.

1. Cut back the leafy fan growth.

2. Dig up the plant with a spade or pitch fork being careful not to damage the plant.

3. Carefully brush away the excess dirt so you can see the plant structure.

4. Locate where the plant has subdivided and use a very clean knife to cut each subdivision into a separate plant. There should be some root on each division.

5. Add compost to the new planting area and mix it in.

6. Plant the divisions. The top of the iris rhizome should be above the ground. An iris bulb should be planted 4-8" deep. The fans should all point the same way.

7. This advice on fertilizing comes from Paradise Garden: "Specific fertilizer recommendations depend on your soil type, but bone meal, super phosphate and 6-10-10 are all effective. A light application in the early spring and a second light application about a month after bloom will reward you with good growth and bloom. Avoid using anything high in nitrogen, as nitrogen encourages rot problems."

You might try lifting and moving them they may be over crowded. They also may need more sun. Check the gardening site below it's been helpful to m for many different gardening topics

they sound like you need to dig up and split and replant

in a fresh area