Never had problems with SPG points crediting.

When you make a reservation, be sure you enter the letter (P - SPG Platinum, G - Gold etc) before the SPG numbers. I don't think the letter actually matters but Sixt website will not take it without the letter first. If you have a rental where SPG number was not entered, you can put it in after the fact on Sixt website (bonus check).

Also noticed that no SPG points if renting on corporate rate.