In my opinion it depends, can either be inner problems (shyness, missing confidence, etc) or outer problems (no idea WHAT to talk about).
I am the type of people with outer problems. There is just not so much stuff to really talk about in my life. And this problem leads to silence in a conversation where I just start to tax my brain to find a damn topic. But it's suicide, my brain is filled with thoughts like "why's there no topics!?" and it prevents me from getting a clear mind.
I really like talking and joking. With a friend I can talk hours and hours about PUA. If I was interested in cars and motorcycles I could chat forever with other friends about that, but unfortunately it's more than just boring for me

I was into computers and programming, and I was able to talk weeks about it with those who share these interests. But in my society it's a
huge DLV to even mention programming/computing, even if you make it sound interesting.
Then, some days ago in school we calculated lottery stuff. And I came up with "What if you won the real thing? What would you do with 130 million €?" And I could have a talk with my friends about it for more than half an hour. Everything people mention then you can use as a new thread to another discussion. Later I asked my gf the same and we could talk 1 and a half hour about it
I will investigate such questions further, everybody can identify themselves in such situations and keep up a nice conversation.
But in general I would advise you to get a range of different hobbies to have some stuff to talk about... I do inline skating (if it wasn't starting to become cold now..) and go to the fitness centre, and I start to go climbing.
But I miss some wicked activities which stand out. It's just lack of topics to talk about
