I guess it's the same as all of the bad habits we develop over time without even knowing what we're really doing to ourselves.
You are a perfect example of people’s everyday problems they can’t figure out what is causing them. It’s like in my profession. I work with electricity, and what happened to you is happening to me on a daily bases. At first I was really frustrated when I couldn’t find the cause of this particular problem, however I had always solved it. I look for every litle detail and eventually I always solve it, no matter what. The most important thing is to never give up and never panic, event if you have to look 100 time I guarantee you, eventually you will find it. Usually it is it, just like you mentioned “must be something blindingly obvious”