It's a lot easier to focus on the hard problems. When you really need to put your mind into it there is nothing that can stop you. As long as you can break the problem into bite-sized pieces so it won't be overwhelming.
But all those easy things. Easy enough to think that you can have them done in no time. Things that don't even warrant a for planning.
They might seem easy, but there is always some depth to them. But it occurs to you too late. Only after you have already gotten to a state of mind where you don't think too much anymore. Suddenly, when there is some minor obstacles it feels too hard. At least compared to what you expected. And you weren't ready for that.
Or, it could be it is indeed as easy as it seemed. Nothing to worry about. No need to pay attention at all. Just do it routinely. It's hard to pay focus on such trivial things when you could be doing something more challenging, more suitable to your talents. You get bored and lose focus, whatever little still remained. And then, with the lack of that needed attention, you fail those easy things.