Why are some photos aligned, but similar ones are not (with NA next to them)?

[Brit]

Why do some photos process as aligned, but similar ones are not (with "NA" next to them)? Can they be improved to align? I am attaching two examples. I see that the masking is not perfect, but that should make a difference should it?

[bigben]

I'm not sure of the reason why, but you can select them and then right-click and choose "align selected images" (or something like that). You will see points appear in the sparse cloud for these images if they align at this point.