I posted this as SE but hoped someone here might be able to help also…
I rendered a car with an alpha background and road surface set to shadow catcher which worked fine (top left pic) but when I replace the first car with a different one the shadow catching doesn’t work for some reason(top right). There is a hint of shadow around the car but nothing directly under it. In fact where the shadow should be darkest is solid alpha. As far as I am aware I have the same settings in exactly the same scene for both renderings of the 2 vehicles.
I used white background to show the shadows that exist so the pure white is actually alpha. 
The bottom pic shows the shadow working with the ground plane but I can’t work out what is different in the alpha background version for the 2nd car. I have duplicate files and checked and double checked the render set up correctly and as this is something I have done several times before successfully I’m sure I haven’t overlooked anything obvious.
I’m thinking I have either changed a setting somewhere without realizing or less likely,the 2nd car has something different about it. Both cars were freebies from turbosquid and I have used them both with shadow catcher previously.