If you’re syncing your source files but not seeing screens generate, you may need to check your export settings. If you have an export mode defined for an artboard in Sketch and that export mode exports to PDF, SVG, or TIFF then InVision will ONLY export that artboard to your assets/images folder and NOT to the screens folder.
By default, you don’t need to use any export mode to sync your screens. If you do need to export a PDF, SVG, or TIFF version of the artboard then you'll also need to add a PNG or JPG export mode if you'd still like to export a screen as well.
If that doesn't help, please contact us and let us know the name of your Sketch file and your project so we can look into this for you.