Some irises, such as Japanese iris, like water. Others, such as Siberian iris, prefer quick-draining sites. I take it you have the latter. A few things that will help flowering: make sure the site gets 6 hours or more of sunlight a day. If the soil is clay, amend with organic matter to improve drainage. You're doing the right thing by making sure the tops of the rhizomes (root structures) are above the soil line. Another thing you can do to encourage blooming is thin your stands out every 3 or 4 years, dividing the rhizomes and spreading them out to give plants more room. Iris don't need much fertilizer. If you do give them anything, use a side dressing of bone meal in early summer.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine