I was struggling with the same issue the other day.
Solution is pretty simple.
I assume that you have assigned Header partial design to header
placeholder and Footer partial design to footer
placeholder in respective partial designs.
In Page design therefore header
and footer
placeholders are not available anymore.
I believe that in Page design you have assigned Page Content component or something similar to main
placeholder.
When these three things are like this, when you are in Experience Editor you won't see "Add here" anymore as all placeholders are "taken".
To enable something in main
placeholder, put there Row splitter component in Page design and move everything that you had in main
placeholder to first row.
Then you will be able to add new content in second row of that splitter.
More information here: