female pekin with diarrhea but vet meds not working

My adult female pekin duck named Berkeley has diarrhea for several weeks. losing weight. she has as 12 or more poops a day. some are mostly normal but many are very runny. various colors reflecting what she is eating. she is eating well(crumble and greens) plus some celery in morning and some tomato in evening. I put some rolled oats and sometimes cracked corn in the crumble. I also add some grit for her. I give her some oyster shells on the side since she is a layer, diet had not changed when this began. I think the only difference was new bag of nutrena layer feed which she had before. the vet says the tests show bacterial infection. kidneys are fine, no foreign objects found and she is laying normally. vets have tried several antibiotics-septra SMZ/TMP in suspension, nystatin, and Ceftazidine by injection. The Ceftazidine helps a bit but not much.
the vet won't even discuss probiotics-she barely asked what i fed her. will probiotics help? could the bag of Neutrena have tainted feed? I bought it at a feed store.
Please help.

 

thank you. will give it a try. also heard that organic apple baby food could work. she does not like yoghurt.

 

I am getting new feed for her. i am feeding her greens, rolled oats, some cracked corn. she loves bananas so i will give her more. I don't think the water is a problem.
she is laying an egg every day so she is not eggbound. She has had a few ultrasounds that show no blockage or egg binding.
I will talk to the vet about Bactrim.
thank you.

 

I took some pictures a few days ago that I can post. I have to figure out how to do that. I am new.

 

ok. the only problem with the poop is that I have many different poops in a 2-3 hour period. I will post soon.