Hi Steve

Thanks for reaching out on Lucidchart Community! The reason that text is not highlighting is likely because the Line Height under Text Options has been edited to 0. This can be done for individual shapes groups of shapes or entire pages on docs. I have created a gif below to explain that even if you reset the settings for a page (by changing the Line Height to 1 if it is at 0) it will only apply to new text boxes and not any that were changed prior. I would recommend using Ctrl + A commands on your keyboard to select all the text boxes on your page and then type 1 into the Line Height (if at least one of them is at 1 it will show 1 even if all the rest are set to 0 so I would recommend trying this even if it looks like it is already set to 1). Then while no shapes are selected make sure to set the Line Height for the page to 1 to make sure any new text boxes will show highlighted text as well. 

If this fix does not help there may be a doc specific issue that you are seeing. If selected text is still not showing as highlighted after you have followed the steps provided above please provide a screenshot of the issue in the thread below!