Q: A custom dimension showed '(not set)' on half our events — how was the cause found?

Q: A custom dimension showed '(not set)' on half our events — how was the cause found?

A: A store registered a coupon_code dimension but 52% of purchase rows read '(not set)'. The parameter only existed on coupon-applied orders:
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DebugView: filtered to purchase events and inspected the parameter panel
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Saw coupon_code present on 48% of events, absent on the rest
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Updated the dataLayer to always push coupon_code, defaulting to 'none'

After the fix, '(not set)' dropped to under 2% (genuine edge cases), and they could finally segment purchases by coupon use — revealing coupon orders had a 23% higher refund rate.

TL;DR: '(not set)' usually means the parameter is missing on some events, not a GA4 bug. DebugView shows exactly which events lack it.

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