It's an old topic, I just occasionally stumbled on it, and since it's still now answered, decided to throw in my two cents.
It's not exactly clear to me under what conditions you're encountering the behaviour - e.g. in what mode you are editing the stylesheet (Styler - Edit Stylesheet or Edit - Edit Resolved Document - For Styling), is it a new stylesheed developed from ground up or one built from the default / OOTB stylesheet, so I have a few educated guesses / suggestions, depending on..
1) If you're editing via Styler - Edit Stylesheet, by default only tags available in the currently opened doctype (DITA Map in your case) are visible and tags from the doctypes included aren't. You can make all tags available in the stylesheet visible by disabling View - Only Elements in Document / View - Only Elements in Document Type in Styler.
If it doesn't help, check #2.
2) If #1 doesn't help and / or you're editing via Edit - Edit Resolved Document - For Styling (the preferred way), and the element is still not visible in Styler, you may have accidently deleted the elemts.
In this case, just put your cursor inside the element missing in Styler and select Insert - Element in the Styler menu.
On the screenshot below I've intentionally deleted the "Title" element from the stylesheet, so it's not available / visible in Styler.
After clicking Insert - Element the element will be added and actually styled based on default stylesheet setting for this kind of elements ("Title" in this case).
Check the Help Center topic 5857, and adjoining ones, in case you haven't yet, for more details: https://support.ptc.com/help/arbortext/r8.1.1.0/en/index.html#page/styler%2Fstyler%2Fhelp5857.html
