well I ve been pooh hooed before for saying it...but will do so again.
my first bird dog was taught to retrieve with black birds on back lawn.
pigeons or plovers are in-between size.....
will she retrieve tennis ball from water???
keep a whole duck of two...smaller hen or better still a spoonie and do bait n switch in back yard when she really keen on the retrieve of whatever else you using....
of take her to professional who will probably force retrieve her eg place duck in her mouth and hold it there..... Ive done that too.....much prefer it to be dogs choice to do it...but if told to do it...do it they must.
you say she found every woundie etc...I take it you then picked them up?????
when playing retrieve DO NOT accept ANYTHING less than delivery into palm of upturned hand....none of the drop at feet BS....none of the chuck it at you then turn and wait for it to be thrown...squat down with hand out and DONT GIVE IN.... when placed in hand..pat and then throw again....
theory being duck will be same as any other object.... also take turns with other dog...and keep calm.
good luck and keep us informed.

Thanks you two for the replies. To answer your question, we definitely picked up all woundys.
A lot of tips there, most I don’t practice.  Haven’t tried a tennis ball on water but will now. Speaking of water it isnt unusual for her to swim around in very deep water for up to ten minutes pushing a duck around and around!
At the end of the day I guess we just have to keep trying.

point I was looking at with woundies.....DONT pick it up...make dog bring it to you.my old bitch took more than a year to pick up bird off dry land...because the boss will do it... then we clicked and never looked back.

Thanks thebomb , however she’s spent hours with three different good labs swimming and following them to and fro from birds.
She’s close, but not quite. As in has no trouble retrieving a frozen water fowl “smaller than a mallard” each night before her dinner.

No problem at all to retrieve the “smaller” bird any where, any time you like. Once again, it’s  just the full size mallard she can not pick up or get her mouth around

lol our recently departed cocker spaniel was the same.... would grab goose by neck or wing and start paddling like stink...hopefully in right direction of shore....
CAN your dog carry something larger???? so next time you doing the fetch thing on back lawn keep upping size of object you throwing....my old bitch used to carry a heavy seat cushion if asked..that thing was about 4" high and weighed a good 10lbs  .if you can get dog used to carrying larger object I suspect your issue will simply disappear.

The dog in my avatar was a great retriever,he even retrieved a Sika hind I had shot caught on logjam from the ngaruraro stream in kaimanawas saved me from getting my boots wet..