How Target CPA works
You give Google a target cost per conversion, and its Smart Bidding raises or lowers your bid in each auction to average out at that cost. It is optimizing toward a goal, not enforcing a ceiling on any single click.
The too-low trap
Set the target below what your real buyers cost and Google does exactly what you asked: it finds the cheapest conversions available, usually low-intent, then stalls once that cheap traffic runs out. Higher-value buyers sit above the ceiling, so the algorithm never bids for them. Google's own docs flag this as "Limited by bid strategy" and recommend raising the target. The fix is to raise it, or switch to Maximize Conversion Value and feed real values so it optimizes for worth, not volume. Full breakdown in A low target CPA isn't saving you money.