(NB: this actually applies to any paid media, but in this case, we're dealing with FB):
STEP 1: Isolate the timeline where the problem is. Recent ROAS decline, for example, like you'll see for one of my brands in #8 here.
Inexplicably, @FBBusiness Ads Manager still does not have CR or AOV as columns on the dashboard. So you have to export cost, purchases, and revenue to a spreadsheet and calculate those.
Pro tip: if the problem is recent, use 1-day click-only purchase attribution only, that way you don't compare older ads (with longer delayed attribution timelines) to newer ones with less time for realized attributed revenue. Otherwise your CR numbers will be skewed.
Steps 2 & 3 go hand in hand: once you figure out which variable is causing the problem, go deeper into why. CPC, for example, is just a calculated combination of CTR and CPM. So if it's up, why is it up? Is it CTR, CPM, or both?
Repeat all of these steps (particularly 4 & 5 at this point) until you solve it.
But look at the CPC: it's way up, correlating with a big CTR drop. And that was in the same time period that we increased spend. So the current hypothesis: audience/creative fatigue.
If that doesn't work, we'll repeat the process.