How to get FindElementByAccessibilityId to find an Id inside a tabitem

I'm using Appium UI testing, and its fine until I try to find an element inside a tabitem. It generates the error 'An element could not be located on the page using the given search parameters.'

I've tried this:

var ucMedusa3D = session.FindElementByAccessibilityId("ucMedusa3D"); // doesn't work      

and this:

var tabMedusa3D = session.FindElementByAccessibilityId("tabMedusa3D"); // works
var ucMedusa3D = tabMedusa3D.FindElementByAccessibilityId("ucMedusa3D"); // doesn't work

n.b. session is the WindowsDriver variable and is set up correctly.

I can find other elements, but not if they are inside a tabitem.