Randomly unable to add blocks to containers

I have a few existing containers within a webpage that I worked on a few weeks ago. Usually when a container is selected, there is a little ‘plus’ icon that appears in the bottom right hand corner of the container. When hovering over the little plus icon, it says ‘add block’. And when clicked, this allows me to add new blocks within the selected container.

However, now when I select a container that I previously created, there is no little plus icon in the bottom corner. It doesn’t appear at all. So now I’m unable to add any more blocks to all previously created containers. However, when I create a new container, the plus icon appears and I’m able to add blocks as usual.

Please see attachments for reference.

Does anyone know why this is happening?

Thanks.