I always smile when one of our customers has a problem and he is "100% sure" it cannot be this or it cannot be that causing it.
In almost every case I have dealt with over the last 40 something years, whenever I hear someone tell me they are 100% sure of something a big flag goes up.
"I am 100% sure there is no network problem." This immediately points me in the direction of looking at the network as the problem. Interesting that whatever someone suggests they are certain it can't be something it's an indication that's exactly the area the problems lies in.
I think this is a normal human trait and one that I get trapped in as well. I was trying to install a new door lock on my front door and for the life of me I couldn't get it to line up properly. I found myself saying "I am 100% sure I have this right". Of course I had it wrong - otherwise it would have worked.
Whenever you catch yourself having this thought process, put it aside and keep it in the background.
You'll do much better keeping an open mind to fixing problems.