Congestion

[Amarie420]

Do you have photos of the inside of her beak or a video of her actions?

For video, upload to youtube and provide a link.

I'm so very sorry to hear she's not improving. It always seems to be a favorite that is ailing, so heartbreaking!

Oxytetracycline can help relieve symptoms from Mycoplasma, but she may have something else going on as well.
It's concerning that your other ones are beginning to show signs of illness as well.
How are you keeping them? Do they have plenty of space and ventilation?

No one can give you a definitive answer on when it's time to cull, you are the one that sees her and can best access what condition she is in. If vet care is an option that would be good.

She literally just passed away

 

[Wyorp Rock]

She literally just passed away

I'm so very sorry!

It's hard losing a much loved bird.

If you feel that you need more information as to what you are dealing with, then it would be a good idea to send or take the body to your state lab for testing. The information you get can help you determine the right course of treatment if any.
I understand if that's not something you want to do or depending on your state, testing can be expensive. https://www.metzerfarms.com/poultry-labs.html

If the Oxy was not making a big difference in treating symptoms, then you may wish to consider using a different antibiotic to treat the others. Tylan/Tylosin is commonly used to treat Mycoplasma. If it's Coryza, then a Sulfa antibiotic would be the best. Baytril is another that some use, it's a very strong antibiotic and not approved for poultry, but sometimes vets prescribe it off label.