Check your Renovation Filter Options. It is likely that you are using a Renovation Override Style that changes the Fill Type and/or Surface for the Existing Elements - see screenshot:
In this screenshot, I have a brick wall set as Existing however it is showing as a simple surface with no hatching, similar to what you described.
The active Renovation Filter is New Construction, and it is set to Override Existing Elements. I opened up the Renovation Override Styles to show the settings for the overrides of existing elements.
In this second screenshot, I changed the active Renovation Filter to 05 Planned Status, and this same Existing brick wall is now showing the brickwork:
To fix your issue, you can either switch to a different Renovation Filter (for example, in the standard US ARCHICAD 19 template you will find 05 Planned Status does not override Existing Elements) or you can change the Filter Settings for the active Filter to Show (without an Override) Existing Elements or you can change the Renovation Override Styles for Existing Elements so that it doesn't change the Surface.