AutoCAD will hatch a selected area using a given hatch pattern but if I switch to another pattern, the hatchure disappears. This behaviour may appear just in some scales and in some areas. Why?
This may be caused by the variable HPMAXLINES - it controls how many line entities can be created in the hatch area. If you hatch a large area in the scale 1:1, a very detailed hatchure with many elements will be created. So you need to set this variable to a large number, e.g. 100000.
The behaviour is described in the following animated image. When you set the variable to 100, the hatchure is not created/displayed, but if you increase its value to 100000, it appears as expected. Used hatch pattern: AR-CONC.
In older AutoCAD versions, see also the environment variable "MaxHatch".