Why don’t the buttons appear on excerpts?

If you’ve chosen to add sharing buttons to post/page excerpts (and having selected to include them on index / archive webpages), but they aren’t showing up, then your theme may be using teasers instead of proper excerpts — a teaser is the post/page text up to a more tag in the content. An excerpt is the first 55 words of your content, or the excerpt text you’ve manually entered for your post.

The WordPress codex describes the excerpt , automatic excerpt, and teaser as:

The manual WordPress excerpt is often confused with the automatic excerpt or with the teaser (the part of a post that appears on the front page when you use the More tag). While both are related to the manual excerpt, they are different from it.

The relationship between the three is this: When a post has no manual excerpt and the post template uses the the_excerpt() template tag, WordPress generates an excerpt automatically by selecting the first 55 words of the post. When the post template uses the the_content() template tag, WordPress will look for the More tag and create a teaser from the content that precedes the More tag.