Are you planting them too deep perhaps? At least half of the rhizome has to be above the soil. I'm not sure what effect the clay soil might have on them?

I take no particular care of mine and they flower prolifically every year though I have to confess that this part of the Czech Republic is particularly good for flag irises. There are some photos on my homepage......

You could have hit the nail on the head Cestina, the rhizomes are perhaps a little deep, I did take a look at yours, they are amazing! I envy you, I love them but they don't seem to love me or my garden, should I lift when dormant?

I don't lift them although every few years one is supposed to do so in order to divide them and make lots more!

They survived a winter of -30 here this year. Unlike the buddleia which died :-(

I will try and post a photo of the miniature ones I have in stone cattle troughs....

Flag iris needs good drainage and sunny position. is it in sun. If not, you will get plenty of leaf but no flowers,move to sunnier spot.

Thanks grandad it looks like I didn;t get anything right with this one, the site where they are only gets sun in the evening, can you advise when I should move them?