I would raise the plant up a little in the water. Also, the lack of
fish may mean that there isn't enough fertilizer, unless you also add
that. The other plants may just not be as heavy feeders. You don't
say what they are, so it's hard to say, but that may be it. I just
noticed that you said most of the others are doing fine. Those that
aren't may also need a little feeding. Others here may know more, but
that's all I can offer, except that if you go on gardenweb.com, there
is an iris forum and a UK gardening forum. Someone over there might be
able to help if you don't get what you need here. Good luck. I like
irises too.