[Pasha Paterson]

To reiterate what Rob has said above (and multiple people have posted previously): If you are in Assetto Corsa or Content Manager, then after you select your skin, and return to the home screen, just right-click and Refresh the page, or re-select Race from the top menu, and you will see that you were registered.

Go back in the Standings and look at the races where you tried to register; if you see that you participated in a race and got a score of 0, then you were indeed registered and just didn't notice.

Yes this is a strange behavior of the UI but you can always work around it as described above.