Publishers like ACM require that only certain types of fonts be used in the camera ready (e.g., Type 1 or TrueType fonts, but not Type 3). Fonts that get flagged usually come from figures. You can run pdffonts on a pdf file (the paper itself, but also individual pdfs for your figures) to see what kinds of fonts are being used. If you are using a flagged font (e.g., Type 3), whatever tool you used to create the figure should allow you to change to an accepted font. For instance, here are tips for making matplotlib use TrueType fonts.