Performance dip — diagnostic post-mortem

Help me diagnose a performance dip in a marketing channel.

Channel: Meta (Facebook/Instagram)
What's dipped (metric + by how much): {{dip}}
Over what time period: {{timeframe}}
What changed in our setup recently (new creative, audience, bid strategy, landing page, anything): {{recent_changes}}
What's stayed the same: {{unchanged}}
External context (seasonality, competitor activity, news, etc): {{external}}
What I've already checked: {{checked}}

Walk through likely causes systematically. Order by probability based on what I've shared. For each cause:
- 1-line description of the hypothesis
- How to confirm or rule it out (specific check)
- What to do if confirmed

Cover: creative fatigue · audience saturation · bid / budget changes · attribution / tracking issues · landing page issues · seasonality · competitor pressure · platform algorithm changes · audience demographics shift · macro factors.

Close with a recommended order of investigation (the 1-2-3 to do first).

Plain English. 350-450 words. Be specific — no 'optimise your funnel'.

Return the full diagnostic.