Hmm - don't know, can't replicate that. E.g. the citation below is created by selecting the respective citation in Zotero, right-click, create bibliograph, select "American Sociologal Association" and copy to clipboard, then paste here:

Gerring, John, Phillip Bond, William T Barndt, and Carola Moreno. 2005. “Democarcy and Economic Growth. A Historical Perspective.” World Politics 57:323-364.

Each style has a defined way of dealing with multiple authors - some abbreviate more than three to et al., some only do that in citations but not in the bibliography etc.

One thought: maybe you have Authors put in the wrong way? You should see 4 lines, each of which says "Author:" right below the title of the respective work.

As for how do you change this, even that is possible
Open:
chrome://zotero/content/tools/csledit.xul
then select the style you want to change. Let's say we stay with the ASA style. Somewhere pretty far down, you'll find these lines:
<bibliography>
<option name="hanging-indent" value="true"/>
<option name="et-al-min" value="6"/>
<option name="et-al-use-first" value="1"/>
Which mean that in the bibliography, articles with 6 or more authors are abbreviated by "first author et al." You can now set this to 8. Or 2 - and actually watch the preview panel (obviously only for bibliography) change.

Important: You should then also change the title all the way on top, so instead of this line:
<title>American Sociological Association</title>
write
<title>Modified ASA</title>
(for example).

Last step - how do you get this new style into Zotero?
You select the entire code and paste it into a text editor.
Then you save the file as ModifiedASA.csl - the important part here is the .csl - and drag it to Firefox, which will automatically install it in Zotero. Done.
For Citations, the same line exists a little further up in the code, so e.g.
<citation>
<option name="et-al-min" value="4"/>
etc.

Edit:
1. as per Dan: Also make sure to rename the ID field (into whatever you want) as otherwise your style will be overwritten with each update.
2. If you have troubles renaming a file as .csl in windows, you probably need to make file extensions visible, see e.g. here: http://www.pptfaq.com/FAQ00781.htm

Yes, you can either:

1. Add the 'suffix' attribute to whatever 'text' element outputs your date (in many styles, a macro does this & you can add it to the macro or where the macro is used, a'la <text macro="XXX-date-XXX" suffix="."/>
   
2. form a new 'group' branch & use the 'delimiter' attribute.

EDIT: I see you are basing your style off of Harvard Leeds. This doesn't use the macro. Replace:
<layout>
<text macro="author"/>
<date variable="issued" prefix=" (" suffix=")">
<date-part name="year"/>
</date>

with:
<layout>
<text macro="author"/>
<date variable="issued" suffix=".">
<date-part name="year"/>
</date>

Hi There
I created the citation style as per Adam Smith's instructions and that seemed initially to work in my document, with for some reason the addition of lots of new full stops. However, I deleted my bibliography and went to add biblio again and firstly my computer crashed, just shut down, then when I went back in to add biblio again the cursor keeps flicking on and off very fast, but no biblio appears. It is like I have no citations. Very confused, very worried this is my finished PhD thesis which I am submitting for examinations in two weeks. Any suggestions? Linda

Two thumbs for team Zotero!! Thank you for the support!!
Had the same issue. Instructions were really helpful.