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The brief was simple: mobile conversion was a third of desktop. The cause was not simple, but it was findable.
What we found
The checkout button fell below the fold on a 360px screen. Everything else was a rounding error.
What we changed
Rebuilt the product page mobile-first, moved the add-to-cart button into a sticky bar, and cut the hero image to a quarter of its size.
What it moved
Mobile conversion roughly doubled over the following month. Desktop barely moved, which is what you would expect — it was never the problem.