I have the same issue when I use Google fonts. @import url... doesn’t work so I use <link href='https://fonts.googleapis.com/css?family=your-font-name' rel='stylesheet' type='text/css'> in HTML sheet instead of @import url.
(when you change the Html code you need to hard refresh the page after you save your change with ctrl + f5 to see the result)
@import will only work if it’s the first line in a CSS file. As the customizations are alphabetically ordered before being combined together, if you happen to name your font loading customization ‘Fonts’ it probably won’t be the first in the list which means @import ... will be halfway down your custom css file when it’s generated.
So as @Trash says, better to use link rather than @import.