The benign explanations: early wins came from fixing obvious breakage (one-time gains that flatten), seasonality, or rising competition. The uncomfortable one, common enough to check, is the onboarding-team switcheroo: senior people launch you, junior people inherit you, and effort quietly halves.
Diagnose with the deliverables ledger: compare what shipped in month one versus last month (changes made, tests run, pages produced. Every platform has change history). Flat results with shrinking activity is drift; flat results with full effort is a strategy conversation.
Either way, force the reallocation meeting: 'here's the trend; what changes next month?' A good agency arrives with that answer before you ask. One that offers a bigger budget as the only idea has run out of them.