Friend problemss?? plzz answer!!?

ok so my supposed best friend used to act rlly close to me always telling me that im her best friend and we would txt frm the time we got home frm school until we went to bed. But lately she hasnt been answering my txts or calls and shes acting much more distant...ignoring my sleepovr invites and having a very hard time getting comfortable around each other. I have been trying to split my time equally between her and my other friend who arent rlly friends and i feel like i ditch her sometimes and that mite be part of the problem. Can someone just give me advice on how to stay friend cz i dont wanna lose her and how to balance friends. Also if this has ever happened to u? how to work it out?!?!? shes very on and off unpredictable...o and we used to wait for each other b4 cl and she nvr comes to my locker much anymore...ugg plzz help me