I feel pretty much the same as you. It's always a toss up as to whether a challenge will spark an idea or just leave me sitting there blankly. I've been doing a bit more lately, but I've been lucky in getting some really great, really unique prompts. And I've spaced them out over almost a year's time, and I tend to be drawn to the communities that are a bit more vague on prompts and have a very wide deadline margin.

The fics that can be written without too much effort are the ones that get thrown out in the idea stage because there's nothing original or unique about them. Hell, I probably hold myself to a higher standard than any challenge community could ever do for me.

Yes, precisely this.