Why don't titles show up in sentence case in bibliographies?
Some styles, such as APA, demand the use of sentence case for titles (e.g., “Oxidation and reduction of iron by acidophilic bacteria”). Others, like the Chicago Manual of Style, require title case (“Oxidation and Reduction of Iron by Acidophilic Bacteria”).
Conversion of sentence case to title case is much easier to automate than the reverse, and this is reflected in the CSL styles that Zotero uses. For example, while the Chicago Manual of Style CSL styles convert titles to title case, the APA CSL styles don't attempt to enforce sentence casing, and print titles as they appear in your library.
If you find that a title is erroneously rendered in title case, make sure that it is stored in sentence case in your Zotero library. You can automate part of the conversion into sentence case by right-clicking on the (unselected) title in the right-hand pane –> Transform Text –> Sentence Case, although this still requires capitalizing the subtitle and any proper nouns.
Moreover, it is possible to prevent title casing of non-English titles.