We are monitoring VDNs and found some of the calls not getting call invalid events.

Can anyone tell like in which condition that events are missing?

Or Is there any settings to get it in every type of calls?

you need to explain your use case more completely.

Are you getting call invalid events for some calls? no calls?
What do you mean by call invalid - the VDN extends the call to a number that is not provisioned? or what?
Are you using the VDN in some form of outbound call campagin?

I can not begin to try to answer your question till what you are doing is clearer.