<b>Q: My custom dimension shows '(not set)' in reports even though I'm sending the parameter — why?</b>

<b>Q: My custom dimension shows '(not set)' in reports even though I'm sending the parameter — why?</b>

A: The parameter is firing, but GA4 only collects it once it's registered, and only going forward.
— Register it in <code>Admin → Custom definitions → Create custom dimension</code>, scope = Event, and the Event parameter name must match your code exactly (case-sensitive).
— Registration is not retroactive: rows before you created the dimension stay '(not set)' forever.
— You also hit the 50 event-scoped dimension cap — over that, new ones silently fail.

Use DebugView to confirm the parameter arrives, then wait 24–48h for it to populate.

TL;DR: Register it (exact name), expect '(not set)' for old data, watch the 50-cap.

Got a GA4 question? Drop it 👇