Try other songs & see how you go. Make sure you're actually dragging them onto the Library icon in iTunes.

Also, go to iTUnes edit menu, choose show options & select date added. Then click on main library, scroll across to date added column & click on top of that column. That will arrange by date added so most recent are listed at the top. This is a good way to check whether they were added or not.

BTW if you already have the identical songs (identical size, name, id3 tag, etc.) in the iTunes library then it won't re-add them.