Importing certain obj files into Leopoly may appear to work, but the application will close or quit as soon as you begin to try and edit the image. Leopoly currently cannot handle invalid meshes and may quit as a result.
One possible workaround is to use Microsoft's 3D builder available with Windows 10 to open the obj file and repair it. Once it repairs the invalid meshes, you can then open the obj file in Leopoly. Note this may not work in all cases.