“Read more” tag disappears in preview mode and doesn’t work?

I created a new page and added 3 blocks. Each block has information on different activities. I want to add a small intro to each of the 3 blocks with a read more tag so that the full info (quite big) only shows up when the user wants to read more.

But, the “read more” button looks fine on the editing page but completely disappears on the preview.

I tried to add the <!–more–> code and changed my reading settings to show only a summary, but on the preview page the code appears as normal text. (Ex. “…experience the best. <!–more–>Tasks vary…”).

How can I make it work??