So when a girl doesn't get back to you right away does it mean shes not interested?

Well, My friend (who've iv been out with quite a few times) tells me to let her know when im free. Now when i let her know i want to do something with her she doesn't respond back right away. She usually responds like a day later or something - or sometimes not at all. Now, if a girl is interested in you and cares for you she would call you intantly, would she not? I thought there was something between us but apparently i miss read her. Stop bugging her? I do have feeling for her when im with her, we seem to be great together but i guess we are just friends hu? She doesnt have strong feelings for me?