One ad set ate 80% of budget and dragged CPA up 19%

One ad set ate 80% of budget and dragged CPA up 19%
Fitness equipment, Campaign Budget Optimization (CBO, one budget split across ad sets by the algorithm), 40d, $130k spend.

CBO funneled 80% of daily spend into the single lowest-CPA ad set — which then saturated its audience and degraded.

Blended CPA:
— Week 1: $44
— Week 4: $52 ▲19%

Moving the top ad set to its own campaign with a hard budget cap, and forcing min-spend floors on the others, pulled blended CPA back to $43.

Read: CBO over-concentrates. Floor your other ad sets or watch one winner suffocate the account.