Running PMax for a US-based Shopify store, and I'm hitting a wall with attribution. Server-side tracking is dialed in-UTM parameters match, campaign IDs line up, and Shopify clearly attributes the orders to Google Ads. Yet Google Ads shows zero purchases for those same orders. Not a few-many. So the tracking isn't broken, but the reporting is lying.

I've been through the usual checklist: checked conversion action is set to primary, no duplicate tags (client vs server firing differently), and enhanced conversions are live with hashed email. Still, the gap persists. Someone in the thread pointed out attribution timing and consent-based modeling, which makes sense. PMax relies heavily on modeled conversions, and if users decline cookies, Google may not credit the sale even if the server-side hit fires correctly.

Here's what I'd recommend checking if you're in the same boat:

Attribution window and model - Google Ads only counts conversions within your set window (e.g., 30-day click, 1-day view). If a user clicks, then comes back direct or via another channel days later, Shopify still shows the original UTM, but Google may not credit it. Compare 7-day vs 30-day windows to see if the gap shrinks.
Consent mode - If you're using Google Consent Mode v2 and users deny analytics/storage cookies, Google will model conversions but may underreport. Server-side tracking doesn't bypass consent signals if you're passing them correctly.
Time lag and backfill - PMax often backfills conversions up to 30 days later. Pull a 60-day report and compare total revenue vs tracked revenue. If the gap closes over time, it's just delay.
Duplicate or conflicting tags - Even with server-side, if you have a client-side Google Ads tag firing alongside, you can get double-counting or conflicting signals. Use GTM's tag assistant or Screaming Frog to audit.
Shopify attribution logic - Shopify's default attribution is last-click, but it may credit the last non-direct click even if that click was days before. Google's model could be different (e.g., data-driven). Cross-check with a tool like Triple Whale or Hyros if you have it.

I'm still digging, but if anyone has cracked this specific PMax + Shopify server-side gap, I'd love to hear what fixed it. This isn't a simple "check your tags" situation-it's deeper.