If I understand the way you captured the scene, it sounds like you did not keep the camera in a single location and pan the camera from left to right, but you started at the left edge of the scene, shot an image, walked a little bit to your right, shot the next image, walked to the right again, shot again etc. This is a problem.
The auto stitch in any application will have problems reconciling the images because you are changing your point of view in a 3d scene. The approach you used works okay for imaging planar surfaces (like a big mural on a flat wall, imaged with multiple shots) because there is no depth to the scene. In your case, let's say that the first car in the garage is facing you, at scene left. In the first shot in your set, let's say that the image shows the front of the car and the passenger side of the car (unless the car is right hand drive, but you get the idea). Then you move 15 feet to your right, or whatever, and shoot your second shot- this time the same car is in the frame only this time you can see front of the car and the driver's side. Uh oh. How does the stitcher deal with this? In reality you can't see both the driver side and passenger side simultaneously, at least to the degree that you can in the individual images you shot. See the problem?
You have to fix the position of the camera, and then pan the camera (rotate it about its no parallax point) to capture the scene properly.
Kirk