Let's say I am writing a computer program, and there is a problem with my code so it won't run properly. Because my code contains so many lines, discovering the source of my problem will be difficult.
Let's say I read every single line of code, and I was just improperly calling a function somewhere.
In this case, the problem I was encountering was very simple, but it was difficult to find.
Another example: Let's say I want to learn how to open my MP3 player so I can replace the parts inside. There may be very little publicly available information about opening up my MP3 player, which makes it difficult to find the right solution, but that solution is as simple as applying pressure to a particular spot and pushing up. (Again, a simple solution, that is difficult to find).