It looks a bit congested to me,and seems like it would benefit from digging some out,especially from under the tree on the left,and just keep a few of the newer pieces..they do need that,from time to time..I think the tree roots and canopy won't be helping,as it seems to be leaning towards the light..I have to keep on top of mine,or I would have nothing else in my border:o).I have never fed mine,and the soil isn't good,in front of a wall,at the back of the border,but they still flower well, ..Someone else may suggest other ways of dealing with the problem...good luck..

They grow a new corm on the top of the old corm each year. The old corm doesn't die off though, it stays connected like a chain of beads.
So you get all these lush leaves, but the most vigorous corms are now closer to the soil surface.
Over the years this gets worse and worse, so you hoik 'em out, dig a bit deeper, put the uppermost corms back in nice and deep and chuck the older parts in the council green waste.
If you put them in the compost you'll have them forever...
Also the soil could be too rich for them, if you have any tomato or rose fertiliser (high in potassium) about the place, you could give them a dose and see if it wakes them up.