In Star Wars: The Force Awakens, Rey can't remember why and by whom she was left on Jakku, but she was also very young.
Was her memory wiped? or was she just too young to remember?
She seems to have forgotten everything important; her parents, friends, why she's there, who left her there, everything.
The implication is that she was simply too young to recall anything distinctly. She has some memory of being left, but a combination of the trauma of the event and post-traumatic stress (life on Jakku is hard) seem to have blocked out whatever was left. There's no evidence that her memory has been tampered with.
Absentmindedly, Rey mimicked the mother’s loving gesture, softly stroking the salvage in her hand. Rey had no memory of her parents. She didn’t know why they had taken her to Jakku, or why they had left her there. For all she knew, they were long dead. Yet Rey couldn’t help hoping that one day a transport would arrive and her parents would walk down the ship’s ramp and back into her life.
When Rey was first introduced and kept insisting her parents were coming back, you could tell pretty much everyone was thinking "Uh-huh. Riiiight." Even she seemed half-hearted about it and eventually gave it up. Once she's accepted that her parents weren't coming back, then it's easier to accept the other memory, that they left her with no intention of coming back and never will.
In short, denial. If she refused to accept they weren't coming back, there was no reason to accept the memory they were nobodies her sold her off and left her behind. Once she got over the first hump, then she could accept the latter memory.
