Can you do us a favor and take a picture of the amount of medication your vet is prescribing using the syringe??? (don't *need* to have medicine in it, just pull the plunger out to the amount recommended)
My cockatiel, Faye, was just on Clavamox.
Clavamox
5.6cc water
0.1cc=8.3mg
Dosage was 0.1 cc twice a day.
She had a bad bacterial infection (i.e. 80% of a fecal smear), but is now mostly over her infections. She has lost weight, but not because of the medications.
It is possible, as the others are stating, that your bird is just not reacting to the meds and needs to be put on something else.