Nymphaea forms buds but doesn't emerge

One of my pond water lilies is behaving oddly.

About 1/3 of the flower buds that form never break the
surface of the water.  From the crown, the stem grows up, 
but then curves downward, and again up making an "S" 

Several inches below the water surface the unopened bud
remains, and then rots without ever emerging.

I forget the particular cultivar.  It's a hardy Nymphaea
with a brilliant yellow flower that emerges in the 
morning.  I chose this variety because of it's ability
to flower with a minimum of about 4 hours sun a day.

In all other respects, both this lily and the pond in
general are in good health.

Does anyone have any thoughts on the matter?  Thanks.