Pickeral Rush

I have 2 Pickeral Rush in my pond. They seem to be growing well but have
virtually no flowers. They develope the little corn cob thing - then just a
few tiny maybe 1/8 inch purple 'flower leaves' will emerge - not even enough
to fall off - then its done - goes to brown. The could be root bound
because it had quite a root mass when I planted and they are considerably
more profuse now. It could be a nutritional problem. I have been
fertilizing monthly but perhaps my fertilizer is not very potent, as I have
posted about lilies yellowing prematurely with same fertilizer. I did plant
the rush with only about 20% soil and the rest pea gravel. Or it could be
just the variety. Met a guy who was given the same plants and he says his
do the same thing. I emailed the benefactors but no response. Water is
good with a small amount of nitrates. Lotsa fish.
What do ya think? Variety or aquaculture?
Thanxx
Bill Brister