Calling someone babe or not isn’t a big deal, i never called my last partner babe. It’s a generic nickname, but she may also associate it with her ex, and calling you it may just not feel right to her because it makes her think of him. There’s other nicknames, and being together for a small period of time is still being in the ‘honey moon phase’, which wears off after 3-6 months. If you’re getting fed up and mad because she won’t simply call you this during the short period of time you’ve been together, I don’t think later on this will necessarily work for you both.