APA style has an unusual rule about capitalization of titles. In APA style, the item title (book title, article title, etc) has 'sentence-case' capitalization: meaning that the first word of the title is capitalized, and the first word of a subtitle (following a colon), if there is one, is capitalized. All other words in the title are not capitalized unless they would normally be capitalized in English usage (place names, people's names, etc). No automated system can determine when certain words are being used as proper names or not (for instance, the word 'new' will often appear in place names, but at least as often will just be the word 'new').
How does Zotero handle this? It punts. When you save a citation to Zotero, it accepts the capitalization that was used in the data source. When generating a bib, Zotero will apply the capitalization rules of whatever citation style you have chosen -- unless you have chosen APA. In which case Zotero changes nothing -- it simply renders the capitalization currently in the title field in the Zotero panel. The data source that populated the title field by default may have used APA capitalization, but most likely it didn't. So you usually have to manually change the capitalization.
There are several ways to do this.
1) Change the capitalization in the title field in Zotero itself. This way you only have to make the change once, and then every time you output that item in APA, Zotero will output the title correctly. When you output in other styles, Zotero will still automatically change the capitalization according to the rules of the other style you have chosen (this works for most other common styles).
2) You can output your bibliography as is, and then edit the titles in your word processing document. This is an ok approach if you are absolutely sure that you are only going to output your bibliography once. Otherwise you really should change capitalization within Zotero itself.
3) You can get an assist on converting titles to sentence case in Zotero by right clicking on the title in the Info pane and selecting "Sentence case". For geekier folks, there are scripts you can run that will change capitalization within Zotero so that it is much closer to APA style. You'll still have to fix a few things, but it will take less time. (For geekier folks in the natural sciences, you could even try to find or write scripts that would handle genus names correctly . . . )