Q: Traffic dropped after migration — how do I figure out WHY, step by step?

Q: Traffic dropped after migration — how do I figure out WHY, step by step?

Deep breath. A small dip is normal; a diagnosis tells you if this one is. Here's how to isolate the cause.

Short answer: work from technical to content, ruling out the cheap fixes first.

Long answer, in order:
— Check indexing in GSC Coverage. Did valid pages drop? That points to noindex, robots, or broken redirects.
— Inspect 10 lost URLs individually — see how Google rendered and indexed them.
— Audit redirects: are old URLs 301ing one-to-one, or are some 302, 404, or chained?
— Compare on-page elements old vs new: did titles, H1s, or content get stripped?
— Segment the drop in GSC by page type — is it sitewide or one section?

Next step: whichever layer shows the break, fix that first and re-request indexing. Don't shotgun fixes blindly.