It looks like you need to turn on a fill in the window to mask the slab. It is also possible that the window fill is turned on but is an empty fill with a transparent background pen.
Otherwise you might be using a window with no fill option. The SE9 library does have this feature but many older libraries do not. In this case the best option is switch to the SE library though this involves a bit of work. You could also add an empty fill under the wall/window and on top of the slab, but his may require fiddling with the display order, has the problem of creating additional elements to maintain, and is probably more work than just turning off the slab and using a fill for the floor pattern.
