Take a look at the Media dropdown. The style you set will only applied to desktop. In the design editor, you can also choose for which media you want to show the styles. The dropdown is in the Style menu.
Instead of changing the heading for a specific media, set it for all cases and use the media as an exception. (I don't know how it's called in RH11 and I don't have RH11 before me at the moment.)
In addition to what my esteemed colleague Willam advised, note that it's also possible that your topic contains some "in-line" styling that may be overriding the rules configured in your CSS file. To check for this, select part of the text that is not behaving as you want it to. Then change to HTML view and look at the nearby code. If you have in-line styling, you will likely see something like this preceding where things aren't behaving: <span style=.
One way to correct this behavior (if you discover it exists) is to select the line, right-click the selected text and choose "Remove Attribute" or "Remove Attributes". (You may see it either way, depending on what has been done.
Once you remove the attribute(s), the text should then begin behaving as you want.
Thank Guys - I'm a little late in replying, sorry. I've checked my styles and yes, I forgot to go through all the Media and change all the styles so they were the same. I did this and my style still wouldn't change. So I edited via HTML and found the unchanged style. Now it works.