Garmin not counting outdoor stairs.

Why do you say that? Which watch? 

A stair/floor is 10 feet (3 meters). Have you walked stairs that are a multiple of 3 meters and checked? Just 1 stair could easily be missed. 

When I'm outside running and walking stairs are counted with my FR945. 

Hi. It's a Vo2. I climb a series of outdoor steps,52 with small platforms inbetween, My fitbit counted these as 4 flights. Garmin isn't counting them at all. Also, fitbit counted if I walked uphill, Garmin isn't.  It does count  stairs in my house.

Which watch is Vo2? Venu 2, Vivofit 2, Vivosmart 2?

My VA4 counts steps outdoors.'

Went up a courthouse entrance stairs, maybe 14-15 steps,. Recorded a floor just fine.