Cell code content only partially displaying

Hi, I am using Jupyter notebooks as part of an Anaconda installation on a Mac (Catalina). Recently, when I open a longish notebook, some of the cells only display some of the code in them, with blank areas underneath. If I click on the blank area the code appears… does anyone know what is causing this and how I can prevent it? I rely heavily on Jupyter for my MSc in Data Science and cannot imagine it not working properly…

Thanks