On the report create page you have the option of checking a vehicle using the VRM or VIN. If the vehicle being checked is in the middle of having a plate transferred there's a chance that it's previous VRM won't return a check. In this scenario you can use the VIN to pull the report.
Just select the "BY VIN" input as shown in the video below and enter the full 17 digits of the VIN. They enter the mileage (if known) and select the type of check you require, a Trade or Retail check.
The Green button at the bottom of the screen will then be active and you can click it to run your full check.