Excerpts displaying randomly or not at all.

I’ve been using the Arras Theme for a couple of months now. However… something has happened that makes no sense to me.

The 20-word excerpts for various posts are not displaying. To complicate matters they are displaying in some places & not in others. For example, if you view the home page of the site https://learnstuffnow.com/wordpress/ you will notice that some of the posts do not display the excerpts in the overlay in the image carousel at the top of the page but the DO display the excerpt in the summary boxes below. And then, in their respective archives the same posts that display excerpts on the home page do not display them in the archive. (See https://learnstuffnow.com/wordpress/?cat=6 for an example.)

I’m totally lost on this one. Can anybody help?

Thanks in advance!

-James.