It could be also that your plants are getting too rich nutrients and growing lots of tops, leaves, and not making blooms. ? Do you fertilize them? Some plants do like lots of fertilizer, I don't know about these, but maybe someone else will. I do think that is a good idea to put them in a north side for the winter and then move them to the sun in the spring. I see that you are a new member. We also have an iris forum, there are several threads there about Japanese iris that may hep you. Here is a link to one of the longer threads, has lots of info and pictures.
The thread "Japanese iris" in
Irises forum
