You need to indicate what sort of Iris its supposed to be, they vary so
widely in requirements, if you are not sure of the name describe the
plant

I *think* it's a Bearded Iris - it certainly looked like one the last
time it flowered. The flower was purple with a yellow and black
speckled centre.

I was reading on the web that maybe it's planted too deep. This could
be the case as the border gets a lot of mulch and compost so the Iris
is in effect getting deeper and deeper each year. Sound plausible?

Lord0