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”).
Unfortunately, it's not possible to reliably automate conversion to sentence case, since there's no way for Zotero or any other tool to know that something is a proper noun. The solution is to store titles in sentence case and instead automate the conversion to title case. This is reflected in the CSL styles that Zotero uses: for example, while the Chicago Manual of Style CSL styles automatically convert titles to title case, the APA CSL styles don't attempt to enforce sentence casing, and instead 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 and choosing Sentence Case, although this still requires capitalizing any proper nouns.
Moreover, it is possible to prevent title casing of non-English titles.
Sentence Case and Subtitles
Some styles that require sentence case, such as APA, also require that the first letter of the subtitle following a colon also be uppercase (“Age and environmental sustainability: A meta-analysis”). Zotero will automatically capitalize the subtitle for styles that require it. If the title contains multiple colons or dashes, store the main title (before the colon) in the Short Title field to indicate where the subtitle begins.