It's like a mechanic trying to find a problem with your car. Sometimes it's easy for a mechanic to find the problem... you drive your car in with a big hole in your radiator... pretty easy to diagnose and know what the fix needs to be.
Of course there are also the times where your car makes a loud knocking sound from the engine... except it stops when you take it into the shop.
The same applies to problems with websites, but imagine instead of one engine in your car to troubleshoot, you have 10! That's what it's like trying to fix problems with very large / robust websites... it's very complex and complicated. Is it a problem with the server, the database, the programming, the ads, the user's browser, the user's computer, other software on the user's computer, the internet connection, or one of the almost infinite combinations of all of the above? Is it happening to everyone all the time or some users once in a while and only under special circumstances?
Just like a mechanic, the #1 best way to get to a fix of a problem is to be able to reproduce it. The problem is that we often can't duplicate the problem. In the cases where we can, then it might be something that is a quick fix or something that takes a ton of investigation and fixing.