What are the steps I need to take when an author won't answer clarification questions?
I get far more emails than I can reply to, but occasionally I do reply to random emails from people I don't know concerning papers I have written.  The main factors that lead to me replying are: (1) I can see that the person is genuinely interested and has tried pretty hard to understand. (2) The person has a very concrete question that I can easily write down the answer to. (3) I am not overly pressed for time from other obligations.
In your case I cannot tell what kind of email you wrote.  But if it says roughly &quot;Dear X, I am interested in your paper but I don't understand your data collection methodology.  Could you explain it in more detail?  Best regards, Y.&quot; then I would ignore it for sure.  Why?  Basically the person has written a minimal-effort email, and so I assume they probably read the paper with minimal effort too.  And what they want from me is completely unreasonable: They want me to write an expanded methods section just for them.
On the other hand, if the email would show a deeper understanding of the topic, and they would clearly explain their confusion and ask a specific question that I can easily clarify, then I am happy to do so, even if it takes a couple of paragraphs of explanation.  However, such emails are rare.
When you publish a paper, the paper stands on its own, with all its strengths and weaknesses.  A paper does not come with any warranty or guarantee that the author will freely provide all interested readers with further personalized instruction on the topic of the paper.