Yes, it likely is the type of file you are using. Raster images like jpegs and some PDF files can only be engraved. They are created using individual pixels of color.
In order to create cut lines, you need to create a vector path (line) for the laser to follow.
We do that in any one of a number of vector programs like Illustrator or Inkscape.
There are some tutorials that explain the difference between the two kinds of files and how to turn them into something that the Glowforge can interpret here. (They’re just short writeups, but they will save you a lot of time.)
Official Glowforge Training starts here:
And some of the Customer recommendations are here:
That should explain a lot of what you are shooting for, and if you have specific questions, you can always ask here…there are experts in all the major design programs.