No idea what size and depth is your pond

About 2 m x 1 m and approx 3' deep in the middle MG.

Many variables here but without more info :
If the lilly has good leaf growth but no flowers i would be tempted to try another plant as they usually flower reliably for me even after division in spring and in 2ft of water,(no harm leaving original in place for another season if it looks healthy otherwise).
If the lilly has disapeared or looks poorly it may have been planted too deep too quickly or doesnt have enough soil round its roots .

Waterlilies also need at least 6 hours of direct sun a day to bloom. Yellow water iris, not so much. A few yellow waterlilies are also tropical in origin, and won't bloom until the water temps reach at least 27º C.

My own are in a pond against an east facing gable wall and being in scotland get much less than 6hrs of sun per day but flower well none the less.

Have you got a small fountain or fall in your pond which is dropping onto the waterlily? Only they don't like that at all and will sulk. If you have falling water, perhaps you could re-position the lily so it's not being affected? :o)

Our water lily does not get 6 hours of sun a day and flowers happily but it is the small water lily (sorry can't remember its name right now). I doubt a normal large one would do well in our pond which is about the same size as yours Ginellie.

perhaps the lily is too deep. Do you get plenty of leaf? There are different varieties for different depths. if its in a basket stand the basket on another upturned basket/stones etc to raise it by at least 6". then see if that does the trick.

I have several and one is next to the pump/waterfall and it doesnt seem to mind the moving water . Non of them have soil in the baskets, just gravel to hold the plant in place. They get their nutrients fro the waste fro the fish etc.

The yellow iris can be a real thug and mite be taking all the nuetriants the Lilly needs .the iris will grow In soil just as happily . lillies can also take over given the chance . it probably needs to mature a bit more . there are lillies that grow in forest ponds so don't need so much light . you could try lifting the lilly basket up onto some bricks or something so the leaves can reach the surface easier . as said lillies can't tolerate splashing/moving water .

Thanks so much everyone. I don't have any moving water and my garden is south west facing so never in any shade really. The lily produces masses of leaves but no flowers. Often the leaves have holes in them which I guess is water snails. Today there was a lot of action from frogs (I counted 15), the pond was like a jaccuzzi. I guess that wouldn't have done the lily much good. I'll certainly try raising it up a bit.

Id say your lilly just needs time if the leaves are growing at the top ok .