So far the best stuff I have found for all of my plants, including my irises is Mirical Grow that goes in the water, it works wonders on everything. That includes the house plants I bought awhile back.
Do you use the strength recommended on the box?
You haven't much experience if you get the best results from granular chemical stews.. Chemical fertilizers like Miracle Crap are full of salts, etc. You'll get much better results with compost, etc. Chemical concoctions are fine in limited amounts for use on indoor plants but even there they eventually corrupt the soil and you need transplanting to a less salty medium.
Sorry compost people or naturalist.
Miracle Grow is by far the best. I have 3 acres of garden and I and many others swear by the stuff. And I don't have time for composting this and that.
After many years of doing various things, I get the best results by using both composted manure and Miracle Grow. The manure does wonders for soil texture and drainage and Miracle Grow provides the necessary NPK. And it doesn't clog my drip system.
After 40 years I am convinced. Find someone with horses that beds on sawdust. Dig in the oldest part of the manure pile. Top dress garden in winter. That and bone meal have grown me the largest prize winning Iris of anything. My blooms are always the largest in the show.
Wow someone else who like horse manure :) Everything I plant does great in it.