Jump links / using Custom ID

I'm trying to create jump link sections of my website pages using custom ID. 

I have been following the information here: REMOVED

However I can not get them to work. 

Using Client Testimonials as a test, I have made a custom link on the main navigation under About us ("Client testimonials") and entered #testimonials into the Custom ID on the relevant section within my About Us page. 

However, the link is not responding and remains at the top of the About Us page.

Here is my website url: REMOVED

Any advice would be much appreciated, thanks.