Kingdom Hearts 1 Question!!

So i finished all the worlds but Hollow Bastion! Now i been trying to go there and everything but it wont let me! Why! I got all the puppy except the ones in HB soo im not sure that a problem, all th postcards, so thats not a problem either! I just wont let me drive there! And i dont know why! Can you tell me why and how i can fixed it or get there? Please! Thank you!!! =]!