You may need to play with the options a little, and more importantly, your source images need to be comparable (in other words, if you took several screenshots of just green acres, and you expected it to stitch it together - well it will have a hard time because the elements are too ambiguous). It is really intended for actual photos but it works fine for screenshots. Being photoshop there are various settings you can play with.
But something like that Luna image would be no problem for photomerge to stick together as there are no open plains of indistinct material to work with. It saves a lot of time and effort.