Reading about the Vietnam Conflict, the Partisans, the Afghanistan conflicts, and many others, it is a common theme that conventional militarizes have a difficult time dealing with non conventional forces. I have read about the brutality of some ground forces and how much that doesn't help, but I find very little in the way of how any army has successfully counteracted irregulars. What makes this issue so thorny?
Counterinsurgency is a very difficult thing to tackle. You need to have the right strategy, be able to organize a centralized socio-economical-political-military leadership as well as provide a large amount of civic action/socio-economic reform. It is a political battle just as much as it is military and by treating it as a pure military affair have lead to many nations entering conflicts which in the end leads nowhere.
I've written about this topic previously (as you can see by my flair) and here are some previous posts of mine which might be enlightening:
What has characterized past, successful counterinsurgency operations?
To what extent was counterinsurgency actually successful in Vietnam?
Why did the United States of America win the Philippine war but lose the Vietnam war?
Malay Emergency vs Vietnam War - COIN done right?
I am very willing to answer any specific questions that you might have (or anyone else for that matter), since your current question is a bit broad. Keep in mind that we can not discuss any conflicts that happened after 1995 in this subreddit.