Are they getting enough sun? Some iris can handle semi shade, but most need lots of sun. It could be the variety, a few just have weak stems. I have an orange iris that has weaker stalks, with large flowers & extra long stems. I had it planted with tall perennials which tended to flop over on the foliage & shaded it some. The stalks would flop before the flowers even opened. I moved it out into a dedicated iris bed in full sun. It still flops a bit, but not nearly as bad. Of course, I can tell that yours aren't having that problem with perennials, but it does look like they are in at least partial shade?