Cloning is too fast and partial

I’m trying to clone sites with NS cloner plugin, version 4.1.5.3.

Most of the time cloning process takes several minutes and it clones the whole content of the site. But sometimes it takes just several seconds, and although it displays “Cloning successful” but site data is not copied. For example, there should be a lot of posts in wp_*_posts but there is only a “Hello world!” post.

This is happening when I try to clone the current site. Any idea how to fix that?