This question basically spoilers the main Season 5 story-arc.
To the question: In the fifth Season of Doctor Who, we learn
that the cracks we see in the very beginning in Amelias House erase people (actually they erase anything) from history, as if they were never born. This includes other people immediately forgetting all about them.
In the episode Flesh and Stone (05,05) The Doctor says
that Amy became immune to this effect because she was a time traveller. More importantly, she didn't forget about a single soldier (cleric) that disappeared in the crack, while the remaining soldiers immediately forgot about their comrades ever having existed.
However, after Cold Blood (05,09) when
Rory is consumed by the crack, she has to try really really hard, while additionally having The Doctor's support, to barely keep remembering him, despite their close relationship. The smallest distraction (usual TARDIS hickup) makes her abruptly forget about him.
Although later, in The Pandorica Opens (05,12)
after a lot of convincing by Plastic-Rory, he manages to make her suddenly remember him. Still this takes a lot of time.
Did I miss some finer point, why some people are forgotten so easily and some are not? It doesn't seem to be consistent. Especially if we believe the Doctor's explanation: she saw and learned a lot more about time-travel between 05,05 and 05,09 and after 05,09 (especially in 05,10!). So if anything it should be easier for her to remember people.