Yep, that's the problem. The button in the map editor seems disabled when I try to edit particular maps.
It works fine when I use it on my old maps, but the ones made recently (112 colours, .PNG and a regular .BIT map), do not allow such editing. Hence there's no girder-holes in my maps.

The map is regular size and completely filled up, so there's room enough.

*puzzled*

I think it is related to the # of colours in a map. Only low detailed maps can have generated holes etc.