I have had this problem before and according to another thread it was fixed.
I just downloaded the latest version for a new site where everything should be using the
www.domain instead of non-www.
Appears to work fine until I click a category link - then it pushes me to non-www page.
In another thread omar said to change .htaccess like this:
Usually, after install, in htaccess is added a line like this:
RewriteRule (.+) $1/ [R,L]
On some hosts, that should not have the R, so it should look like
RewriteRule (.+) $1/ [L]
Open your htaccess and edit that line then save changes. That should fix it.
That worked for me. So not sure if it is a host problem or a script problem.
I'm posting this in case it helps others, can be fixed for newer versions, and to remind myself how I fixed it for future reference on future installs.