When you plant a seed in a garden, there's many factors at play - is the season right? Is the climate right for that seed? Is the soil the right type? Are there pests or varmints roaming that might eat it prematurely? Etc.
The garden of thought has analogous factors: Is this a question your brain actually cares about right now? Do you have the background knowledge necessary to work it out? Is your brain calm enough to process that question? Are there distractions or anxieties that disturb the process?
That said, some people truly are stupid. I recently read John Cleese's autobiography, and he tells a story from when he was a Geography teacher...
There was a lad who he was teaching countries and their capitals. Even when given direct attention, the kid simply wasn't able to name any capitals whatsoever. He would smile and nod, giving no indication of difficulties... But he couldn't recall the info even after being told it 8 seconds previously. This particular type of data slid off his brain.
At the end of the term, the kid got one question right on the final exam, probably by accident. Cleese posted the paper in the teacher's room, attracting the comment from one teacher "The sad thing about true stupidity is that you can do absolutely nothing about it".
Perhaps that kid had a genius for engines or something, but he was never going to be able to understand geopolitics. He lacked even the awareness to know that he was stupid (at least about countries and capitals). He would never have asked if he was stupid, because he was truly stupid.
If your friend is ever curious about their intelligence, they're probably ok and can develop the skill of thinking like this.