Post Voting plugin only adds voting if a full content of a post is being displayed but not to excerpts. For example, if a blog page shows the full content of posts on post list, voting can be added there. It cannot be added if it shows excerpts.
Plugins seem to be hooking to "the_content" WP filter only. If the excerpt is served via "the_excerpt()" WP function (as expected), voting buttons are not injected.