iOS UIImagePickerController'view display nothing except a cancel button

hi, everyone,
I have a very oddball thing. I used UIImagePickerController in my app, I choosed one photos ,show it and do something with image, then go back subviews of photos to select other photo. sometimes when I do that many times and go to first screen that show the category of all photos, there display nothing except cancel button on navigation bar, if somebody can give some advice , I will grateful for that. Thanks.

Thanks for your reply, I have checked it, and there were no memory leaks except when I allocation memory for UIImagePickerController which add to window as Application did finish launch, but I have release the UIImagePickerController object in dealloc function.