My battery is new but my car wont start?!?

So I thought it was my battery that wouldn't make my car start every few times a month I would have to jump it so it can start so I was pretty sure it was the battery. So i got a new one but it still acts the same (btw the battery is held down by a rope around the bolts that i connect the jumper on. so its a bit dusty) The radio works the lights work everything works but it just doesn't start. WHAT HAPPENED!? ahhhhh