Anyone who receives a wedding invitation should send back the RSVP card, after all it’s usually fill in the blank and already addressed and stamped. Easy, right? But sometimes RSVP cards get lost in the mail or the guest gets busy and forgets.
In the case where the groom’s parents, or another key member of the wedding like a bridesmaid, doesn’t reply but you know they’re coming I don’t think there’s anything to say to them about it. You could mention that you were still waiting for their reply because you’re collecting them for your wedding scrapbook if you want to.
In the case of parents and grandparents, they may not send an RSVP because they want to keep the complete invitation suite together as a momento.