If your Google Ads ROAS has quietly declined over the past few months without any obvious reason, you're not imagining it. I've seen this pattern across multiple accounts I manage — and after digging in, the cause comes down to three specific platform-level shifts that most advertisers haven't adjusted for yet.
1. Smart Bidding Recalibration After Broad Match Expansion
Google has been aggressively pushing broad match keywords across accounts, often auto-applying them during campaign setup. The problem isn't broad match itself — it's that Smart Bidding needs at least 30–50 conversions per month to work properly. When your match types expand suddenly, your conversion data gets diluted across lower-intent queries.
What to do: Audit your Search Terms report weekly. Add negatives fast. If your conversion volume dropped below threshold, switch temporarily to Manual CPC or Enhanced CPC until you rebuild signal.
2. Performance Max Cannibalising Your Best Search Campaigns
PMax campaigns get priority in the ad auction. If you're running both PMax and standard Search campaigns targeting the same products or services, PMax will take the impression — even if your Search campaign historically converted better for that query.
I've seen accounts where ROAS dropped 35% simply because a PMax campaign was added without adjusting brand exclusions or asset group segmentation. The fix:
- Add your brand terms as negative keywords at the account level for PMax
- Segment PMax asset groups tightly by product/service category
- Use campaign-level negative keyword lists (now available in PMax)
- Monitor the "Insights" tab in PMax to see what it's actually targeting
3. Consent Mode v2 Gaps Causing Conversion Undercounting
If you have any EU traffic or your site updated its cookie consent banner in late 2025, there's a good chance Consent Mode v2 is not implemented correctly. Google's modelled conversions are filling in some gaps — but not all. Your actual ROAS may be higher than reported, or your bidding strategy is optimising on incomplete data.
Check your Google Tag setup in Tag Manager. You should see gtag('consent', 'default', {...}) firing before any other tags. If it's not there or firing after the page load, fix this first.
Bottom Line
ROAS drops are rarely random. In 2026, the most common culprits I see are Smart Bidding data dilution, PMax auction priority conflicts, and consent/tracking gaps. Before cutting budgets or changing creative, rule these three out first.
If you want me to look at your specific account, get in touch here.