DRC does not recognize all spacing or crossing violations

Hi,

i have used the Design Technology settings and spacing settings to comply spacings and cross overs in my PCB layout. I designed some circuits which are acceptable and directly next to it some which should not. After executing the DRC to check the displayed mistakes i realized, that the DRC does not identify all of them. For example i am getting only 13 violations instead of minimum 29. 

Hope someone can explain or help, thanks

Mo