I generally only get 20-30% bloom on first year irises in my garden. This year has been a particularly good one at 34%. I personally wouldn't get worried with 25% bloom, but I might expect better than that in your zone with a longer growing season.

There are a lot of things that can cause non-bloom other than recent transplantation. You can find the other most common causes in the FAQs (link at top of forum page).

You shouldn't just remove the extra soil from on top of the rhizomes because that will create a depression on top of the rzs that will collect and hold water against them, and THAT is very likely to lead to rot. If your rhizomes are more than 1/2" deep, you should replant them closer to the soil surface. Also, you should not mulch over the tops of iris rhizomes. If you must mulch, keep the mulch pushed back away from the rzs so they and their surrounding soil can dry out between rainfalls.

Are the rhizomes from which your failed stalks grew still solid and healthy feeling? If they are spongy or mushy, you've got rot. If they are firm, your stalks probably failed as a result of some weather phenomenon: late freeze, excessive heat, excessive rainfall, etc.

Laurie