Hardie boards (cement fiber) siding butt joints popping out

Those boards have been installed probably more than 15 years ago. Location is Minnesota, so harsh winter. Overall the installation (still) looks good, but after this last winter, I've noticed that two boards, right under each other, are "popping out" at the butt joints What caused this and how do I go about fixing this? I think probably the blind nails didn't hit a stud there.

Why can't I attach an image? https://i.sstatic.net/YFpygMyx.png