po files not imported, no errors shown

I've added the spanish .po files to one of my sites. My problem is that the .po files are not being imported. I can tell from a quick look at the admin menu that strings like "Site configuration" remain in english when they should be in spanish. However, a number of strings are translated, while lots of strings remain untranslated. I'm pulling my hair out trying to figure this out.

I have two copies of the site, but with the same files (but different databases, and on different servers). In my local copy, I see Configuración del sitio, while on the live site, I see Site configuration. I've followed the same procedure on both sites, deleting the spanish language then adding it again. On both sites Drupal appears to import the po files without error. Yet on my local copy all the strings appear to be translated and on the live server only a much smaller number of the strings is translated.

Can anyone help me track down the problem?