Why have duck numbers coming to our flight pond dwindled over the years?

The most likely reason is that your pool has become less attractive. It may be that surrounding trees or overhanging branches have grown in, making it harder for the ducks to flight in and out safely. Another possible problem is that there is less shallow water for them to feed in. This may be because reeds and rushes have grown into the shallows, forcing the birds into deeper water. It may also be that the ducks themselves have eroded the shallows away by feeding activity, in which case a bit of regrading with a digger is called for.