cream legbar hen mystery illness

Last fall I added two cream leg bar hens to my flock, they were about 4-5 months old- we went through the winter and one has fully matured and being laying regularly for me. My second one has been slow to develop, is smaller in size, her comb finally turned red and got larger a couple of months ago, but NO EGGS yet. This hen is now about 10-11 months old. She has had an on and off sneeze since we got her, and watery stool. But other than that, no signs of illness. No other chicken we have got sick, her vent is clean, feathers in good condition, eyes clear. She was dewormed when I brought her home, and I have recently treated her with oxytetracycline and isolated her. It's hard to imagine what she may have since no other chicken in my flock has gotten ANY of the same symptoms. I'm now wondering if she just isn't getting an adequate dose of the oxy- any tips on how to safely administer these kinda of meds other than in her water? Any thoughts or similar experience on what could be going on with her?
(the picture was taken when we first got her)

Ive had not one. But TWO diffrenr crested cream legbars die on me diffrent ages. Diffrent breeders. One i stupidily paid $80 had her two weeks. She was suposted to already be laying. Never laid. The other had a couple weeks kept close eye on her. Wham dead one day. No signs why same as the other one.