Change Font In Question Bank

As Jesse pointed out, changing the Master layout will adjust the fonts on the corresponding slides. At least, it will if you didn't edit the font directly on the slides. 

In essence, editing the font on the slide tells the program you don't want to follow the Master layout. So reapplying the layout won't work.

However, there is a quick way to change fonts throughout a course. Use the Replace Fonts function. Access that via the Find/Replace dropdown menu on the Home ribbon. 

In the Replace Fonts window, select the font you want to replace, select the font you want to use instead, and then click Replace. It's that simple.

Note that it will change all instances of the indicated font. So if you have some slides where you want that font changed and some where you want to keep the font, you will have to make the changes manually.