problem of mask with shapefile from slice assembly.

masking from slice assembly using shape file is not working. I am sure that its not the problem of shapefile. Because when i tried with individual image at sub district level the result appeared. since my study area (visakhapatnam) occupied over two images, i had to do slice assembly. But masking with is not working. I mean the result appearing just shapefile. After i turn off the vector data from layer manager there is nothing showing.