DAM Cache

In our environment, after content update , the stuff(Pdf/PPT) in cache is not getting invalidated. And thus we are ending up with old content(PDF/Docx). And to work around this we end up clearing the dispatcher cache manually everytime. What could be the issue and where do I look at ?

I have checked the below articles

https://dev.day.com/docs/en/cq/current/deploying/dispatcher/page_invalidate.html

http://helpx.adobe.com/cq/kb/HowToFlushAssetsPublish.html

Thanks,

Prem Pratick Kumar