Why doesn't my job offer us full time work?

I work at next in plymouth, it is a clothes store. I am asking the admin staff and everyone whether there are full time positions available and they always say no. I ask nearly every week!! But now I have just found that they have actually recruited all new staff about 3 times since I began asking!!!! I have heard apparently they don't offer full time hours to people who already work part time but why not? I have already been trained I know how everything works and all the people there....why do they lie to me about not having any jobs open when Iook on the website and they do?? thoughts?