Start with the account structure, not the creatives
The most common cause of a CAC increase that's hard to explain is audience overlap — campaigns and ad sets targeting the same people, bidding against each other. Before you look at creative, pull an audience overlap report. If two or more ad sets are hitting 50%+ overlap, you're competing with yourself. Consolidate and re-run before you make any other change.
Check creative age
Ad creative has a natural lifespan. For most audiences, frequency above 4-5 per person per month means the creative is exhausted — people have seen it, they're not responding, and the algorithm is running out of options. Pull a breakdown by creative asset and look at frequency alongside CTR. If frequency is climbing and CTR is falling, the creative is the problem, not the audience or the bid. The fix is new creative, not a targeting change.
Look at the budget allocation cadence
Paid media accounts that aren't actively managed drift. Budget flows to the campaign that's spending most efficiently right now, which is often the campaign that's spending on the lowest-intent, easiest-to-convert audience. Over time, the account optimises itself into a corner. Check whether budget is being reallocated across campaigns on a weekly basis — not monthly, not quarterly. Weekly reallocation is the difference between an account that improves and an account that plateaus.
Check attribution configuration
CAC can appear to climb without the underlying economics changing, if the attribution window tightens or the conversion event definition shifts. A change from 28-day to 7-day click attribution will inflate CPA on paper without reflecting a real change in performance. Check whether anything in the attribution configuration changed in the same timeframe as the CAC increase.
For regulated industries: check the compliance cycle
In financial services, healthcare, and other regulated industries, the compliance review cycle is often the hidden bottleneck. If ad copy or creative requires legal review before it can be refreshed, the account runs stale creative for longer than it should. The fix is upstream — building a pre-approved asset library so optimisation doesn't wait on a sign-off queue.