Before OCR'ing it converts to black-and-white using a brightness threshold. Keeping an evenly lit background is particularly important, because otherwise a shadow area can easily fall under the threshold of all-black.
A license or ID will almost certainly have medium-contrast elements in the background that will show up as dark. But if you were able to manipulate the contrast/brightness appropriately in advance, you could probably get it to work.
Can you list your script/pipeline? I haven't had much success (though, I only ran ffmpeg's internal tesseract OCR[0], no imagemagick processing or any other processing in between)
A license or ID will almost certainly have medium-contrast elements in the background that will show up as dark. But if you were able to manipulate the contrast/brightness appropriately in advance, you could probably get it to work.