Sometimes, images with seemingly clear and obvious plates are missed by our CarCheck API. When this happens, the "results" property of the returned JSON will be empty.
If the picture was taken from a mobile phone, your issue may be a rotated image. Mobile phones can store images with a different rotation than you see on the screen. This is based on how the EXIF headers of the JPEG image are set by the phone's gyroscope. Rotated images will usually not be recognized by our API since the license plate is sideways. To check if your image is rotated, try opening it on a computer.
If you confirm that image rotation is not an issue, please check point 3 in Improving Recognition.