On a NextGen-converted page, you may find you can only add a handful of components even though the page already has many more. This usually means your content is sitting in the Banner section, which is limited to a few hero components by design. Here's how to confirm it and move your content where it belongs.
You're editing a page and the component picker only offers a short list, typically image, intro text, title, and slide deck horizontal, instead of the full set. That short list is the tell.
To confirm, hover over your content and widen out until you see the section boxes. Components show as blue boxes with the component name. Sections show as gray boxes labeled "Layout Container". Scroll to the bottom of your content and look for a gray box labeled "Layout Container [banner]". If your content is packed into a banner, that's what's limiting you.
The Banner section is the hero area at the top of the page. In NextGen it's limited on purpose to four components: the H1 title, intro text, an image, and slide deck horizontal. Under the old static templates, extra components were sometimes allowed into the banner where they weren't meant to go, so content built up there over time. After conversion, that content is now stuck in a section that won't accept anything beyond the four hero components. There's nothing wrong with how the page was built. It just needs straightening out.
You'll move everything out of the "Layout Container [banner]" into the "Layout Container [Main]" content area, clear the banner, then switch to a template that fits the page.
Try adding a component now. You'll have the full suite available again.
Before you delete anything from the banner, make sure the content copied into Main completely. Give it a close look. Once you delete the originals, the copy is all you have.
This example was straightforward. If your banner holds components with a defined start and end, like a content optimization container, copy and paste more carefully and verify the grouping came across intact before deleting the originals.