I have used multiple custom fonts in my designs & I have never had an issue. I’m not sure if there is a fix or if some fonts just don’t work, but in all of my “chalkboard” themed watches, I have used a fairly basic chalkboard font. It shows up in Facer Creator & on the webpage page, however, when I sync it to my watch, it does not show up. All of the icons & colors show up correctly & the date is in a script font that shows up. My watch is a Gen 4 Fossil Venture HR.
Not all fonts work well on watch face designs and some do not have all the characters needed. What is the name of the font that you are having a problem with?