I am using Bootstrap modal in my homepage. The content from the modals is not indexed by Google, while almost all other content in that page is indexed.
How can I index modal content?
Since the content of the modal "is hidden" until opened (so invisible for spiders) you could try to duplicate the same content on the index page outside of field of view for visitors but not for Google.
To do this successfully you'd need to use a CSS modal (ie. a script loads container content from the homepage into the modal) or probably a simpler way is to create a separate page with the same content, and include that page link in your sitemap xml. If it still doesn't get picked up by Google then you'd need to add a link to it somewhere buried in the site.
And in case you're wondering, it is not a good idea to try to hide content from users on your page (by using negative divs or text the same colour as the background etc.) and still present it to the Gbot, as Google tends to see that as spam content and will penalise your page for it.