Category Drop down list widget not working

Hi

I am using a couple of differet themes on a couple of different sites.

https://privatedetectivemelbourne.com/ theme Weaver
https://www.bookkeepingadelaide.biz theme desk mess mirrored
Both themes accept the category widget

On both I am using the category drop down list from the widget area in the dashboard.

On both sites when you click on a category in the drop dwon list the link goes to the correct url eg
category = Teenagers and drugs
https://privatedetectivemelbourne.com/category/teenagers-and-drugs/

but is actually displaying the post page ie

https://privatedetectivemelbourne.com/

this is the same regardless of setting either a static page or latests post in the Settings/reading on the dashboard

Any help greatly appreciated

Wanda