I can empathise with you fully — I currently teach undergraduate maths tutorials at three universities, and the huge difference is that these students are able to articulate what they’re thinking.

When dealing with younger students I often need to watch their actions more closely to guess what’s going through their mind and how they contextualise and process the information at hand.

The other thing to keep in mind is how our thinking and approaching mathematics changes as we acquire more knowledge. At the university/college level we’re used to speaking more generally and dealing with much more abstract concepts, but when teaching children it’s more important to establish their building blocks with more concrete concepts (as evidenced by your success with your students) before they are able to make generalisations.