It's alright, I get what you mean, Damek. I have the same problem. A few weeks after watching a Giallo, 9 times out of 10, I've completely forgotten what it was all about. It can sometimes take 2 or 3 viewings to properly absorb some of the more convoluted Gialli. In fact, some have taken a couple of goes to actually figure out what they're about.
The memorable ones stay with you, though. Films like
Deep Red,
Short Night of the Glass Dolls,
House with Laughing Windows and
Blood and Black Lace etc. But this is probably more because they're profoundly exceptional films or have a central premise that is unique and so makes an impression. That's not to say other Gialli aren't good, just less memorable for varying reasons. Many times, you can just say 'the priest did it' and you'd probably be right, even if you'd never seen the film.
So yeah, put me down as one who tends to forget what they're all about, if I can even figure them out in the first place. Maybe I'm just exceptionally dull.