I hate to butt in but I feel your breeder lacks an understanding of mycoplasma if she 1) sold you those birds in the first place if they could be carriers, and 2) told you treat with oxytetracycline.
Have those birds been exposed to the rest of your flock? Even to the point of handling these birds first and then visiting your flock?
To treat MG, you need Baytril (enfloxacin), which is best prescribed by a vet, but you can order it online.
Have you done much reading on MG yet? *If* it is mycoplasma, the birds, if they survive, will be carriers for life and can infect other birds without showing symptoms.
Also, there are several respiratory infections with similar symptoms. Only a vet can give you a 100% diagnosis as to what you're dealing with and only after blood work.
MrsB