Bridesmaid probelms!

I have been friends with Lisa since I was 3years old, and she was the first one I asked to be in my wedding party. She accepted. But everytime I try to make plans to go to the store for her to try on the dress, she is always like I am busy etc. Then she says I did not call her in enough time... I called Wednesday, Thursday, and Friday for this Saturday! Did I give her enough notice? She then leaves this message on my machine saying that I never gave her enough time / notice for Saturday. This is the 2nd time she has done this to me. I am begining to wonder what her problem is. It is not like she cannot afford it , she gladly accepted and was so happy that I asked her to be in it. Any advice? I am begining to think she gets one more try and the third time she does this to me, she will be asked what is the story here.... Any advice? I am so fustrated!