Female Bolivian Ram Trouble!

Hi all!, I'm hoping someone can perhaps give me some advice or help, I have my Juwel Rio 180 up and running and stable, I am under stocked and all my water readings are good.
I have one male Bolivian Ram who is doing well, growing, eating, active and healthy, the problem comes with the female, we had one who we think must just have been weak from the off, very thin, not eating and generally disinterested in life who eventually died some time later we went and spent time looking over a tank in our local fish shop with a view to getting a new one, we chose carefully and picked a healthy active female, on bringing her home she settled in well, eating and being active, but over the last few weeks she has become lethargic, inactive and just like the last female has stopped eating, and seems to be fading away, no matter what food we try she shows no inclination to eat, we have spent time watching them, and the male is not harassing, bulling or chasing her, she just seems to be fading away, and we are mystified and more than a little upset, and have no idea what is going wrong, or what to do!! Please help!!

Thanks so much for that Sue, I didn't think of that, feel kind of bad if that's the case. I feed good quality dry and live food. Don't know if we will get another, maybe getting a couple of girls could be an idea, will have to look into it and have a good think