If it’s only happening to a few posts or you can’t see all items/boxes in your dashboard then it’s your php module starving for more memory.
If you’re hosting the site, edit php.ini and add give it more memory. If you don’t know where php.ini is use the following command:
find / -name "php.ini"
Find memory section and dedicate what you want.
If your site is hosted, then create a php.ini file in wp-admin directory, and add the following line:
If 15 is not good enough try 20 or 30.