DC++ goes inactive, even when still connected/online. It doesn't automatically start downloading files when users come on-line

DC++ never goes 'inactive', failed searches happen often because misconfigured connection settings or because of hubs enforcing rules of maximum searches per some time interval.
If you are able to search manually then the first reason isn't likely but hubs can individually enforce their search rules so it's best to check whether they allow auto-searches or if so then what are their requirements to meet to be able to constantly do it.

DC++ itself also can be set not to auto search for queued files so make sure that
  Automatically search for alternative download locations
and
 Automatically match queue for search hits
is enabled in Settings/Downloads/Queue pane.

Also you can increase
 Auto-search limit
in Settings/Experts only pane if you have many sources for the same files.

eMTee also suggests this article as an answer to your question:
FAQ #90: “I can't connect / lost registration or am banned from one or more hubs or I have a problem with a rule, user, etc.. in a specific hub. Can you help me?”.

thanks - but its not just related to auto-searches. when this happens, even when i manually click to download a file (or filelist) from an active user it tries to connect but times out (i think it has even happened as someone else is uploading from me). Again a simple close + reopen seems to fix it but still have the problems mentioned above?
could it occur if there is temporary outage in internet connect (even for a second or 2) due to wifi issues?

Sure, if your your IP (either external or internal) is changing after the outage.
The Troubleshooting part in https://dcplusplus.sourceforge.io/webhelp/faq_portforward.html explain this in detail and indeed if your connection is often drops, either locally (from your wifi router) or in the ISP level then you might better set up your IP/ports manually to constant values in order to have the same constant network configuration between outages.

thanks I will have a look. I've just changed to a powerline LAN. I also realized that I restore the connect (without close + reopen) by going to Settings/Connectivity and clicking Detect Now (i have Let DC++ determine best connectivity settings ticked on). whenever i have outages though my IP remains the same (it changed once a few months ago) but has been constant for a while.

So it's probably the internal/local IP then, which you can set to a fix value or if it's not possible then you can set up a static DHCP in your router as described in the above FAQ.