Well, working by your advice I have replaced the letter box with a bigger one, and it worked pretty good with the female, she's always been smart and clean, however, the male was always such and idiot and I wasn't surprised he still ain't gonna cover his nuclear bombs, so I am not very...
So I got to terror them to feel unsafe 128512128513, thanks Mr Columbine for the advise, I was thinking just about the thing lately and you helped me alot, thanks for every one replied.
I had my cats when they were 2 months age and they were covering their land mines just fine, by the time and as they grow up, the male one started scratching the sides of the litter box instead of covering his things, and a while after the female started to do so, now they are about one year...