Problems With First Photos

When I tried to place my first photos together, I found that the images sloped downwards when I place them next to each other. This meant that the two images would not meet up at either end, and cropping the picture would leave very little of it left. The picture below shows my Photoshop document, with rules to show how much of the picture would be left if I cropped it.

Because of this, I have to retake my photos. I also chose different locations for my photos as I found taking photos in sloped areas too difficult, as I was having problems with my tripod even with a spirit level. Because of tripod problems, I had about 6 sets of sloped photos before getting panoramas that fit well across the whole area. I will also pay close attention to the tripod to make sure it doesn't change height so the pictures are taken on a completely flat surface.