Presumably you’ve already checked that nothing is adding a “noindex” robots meta tag to these pages. I can’t think what else would cause this. Google doesn’t guarantee to index all pages, but it seems strange it would suddenly cease to include one like this.
Have I understood correctly that you also have a Google search on your site, and it’s behaving in the same way? It might be better to move this topic to General Web Development, and see if anybody there has any ideas. Let us know if you would like it moved.