FB campaign strategy: I’m using VO with high quality Lookalike Audiences but ROAS is still declining. Why?

LaL audiences sometimes don't pair well with VO campaigns: VO campaigns require a very wide "surface area" of potential users in which to explore. The way VO works if it's not min-ROAS constrained is that it just bids whatever it needs to in the auction to acquire what it deems to be the "highest value" users; it's entirely possible that the bid used might not back out to a positive Day-X ROAS for you.

I'd suggest some combination of:

• Using a min-ROAS setting to ensure that ROAS falls within your standard (note that if you are currently not meeting that standard without min-ROAS, then setting the min-ROAS value will likely just decrease delivery);
• Increase the scope of your LaL audience with either a significantly larger seed audience or a significantly larger LaL percentage;
• Don't target a LaL audience and give VO the loosest possible (eg. Tier 1 countries) audience targeting parameters in which to explore.

More thoughts on this:

• MAI vs. AEO vs. VO campaigns: building a mobile traffic strategy on Facebook
• Understanding bid strategies and cost control in Facebook advertising