While this could be for several reasons, the most common reason is that your WordPress permalinks got messed up somewhere along the way and you're receiving Mixed Content warnings in your Browser's console.
OMGF uses the (core)
siteurl setting (stored in the
WP_CONTENT_URL constant) to generate URLs for each font file. Please check your settings under Settings > General, WordPress Address and Site Address. Make sure they both start with https.
If that's the case, then head into Settings > Permalinks and click Save Changes. This will flush all cached permalinks and rebuild them.
After that, go back to OMGF's settings screen ( Settings > Optimize Google Fonts) and click Save & Optimize to make sure OMGF (Pro) uses the fresh permalinks in its generated stylesheets.
A subset of a Google Font is basically a (sub)set of characters which covers the alphabet(s) of the language(s) your website is written in.
By default, OMGF is set to use the Latin and Latin Extended subset for the font files it downloads. This setting covers all of the American and European continent, so it's safe.
For better performance, you can choose to only use the subset which applies to your website. However, if you select a subset in which your Google Fonts aren't available, system fonts will be displayed.
If your website is written in English or a Western European language (e.g. German, Dutch, Spanish or Italian) Latin will suffice, the further you go East (e.g. Czech or Greek), you'll need to select Latin Extended as well.
Latin Extended is an addon for Latin and should not be used on its own! OMGF v5.3.8 and higher no longer allows you to just select Latin Extended.
If these solutions don't work, enable Test Mode (under the Optimized Fonts tab) and contact me.