Thank you both for the answers. I discovered a way to make search work in dolphin: clicking on "Find" opens a few tabs with some settings for the search options, and then I select "Everywhere" instead of "From here (user)" , as bvbfan suggested. I guess that in another KDE distros the default setting is Everywhere, and this is why I was confused about not getting the results I wanted.
All is clear, now.