Captions not showing ‘plus’ sign

I'm trying to test the 'caption' feature. But I could not add a new caption as the '+' is not showing (in the white circle area attached). Even when I click on 'add new captions segement', there is nothing happening. Please can anyone help me out? Thanks. (attached a comparison of how it looks like on other people's PR, especially in the yellow circle area).

Thanks!