Many of the WordPress themes (even some really good ones from Envato Market) do not have the necessary styles to make the tables responsive. I often see clients coming with a purchased theme complaining that the pages are not behaving as they should for smaller screens. And most of the times I find that the problem is caused by the presence of a table that lacks the responsive styles. The following two snippets are the CSS fixes I use to solve this problem. If you just want to add responsive capability for the tables inside your WordPress page editor, then use the Basic one. But if you want to make those tables look more modern and eye-catching, then use the Stylized one.
Here is an example of how the stylized table will look like:
I hope this helps you in some projects!