New Garmin FR955 solar - solar charging doesn't work

What do you mean, and how do you know, it doesn't work... it's not as if you'll see it increase, unless perhaps in full sun on a minimal watchface not doing anything.

I noticed the battery was depleting quicker than expected - so I left it on an outdoor table in full sunlight to see if the charge increased at all - there was no increase over several hours

Where is written that solar can charge the watch? It can only prolong battery life a little bit, but it cannot charge it as power drain is higher than solar charging can provide.

In my opinion solar battery works well. I spent two weeks in Mexico, almost all days outside. At the start of the trip the battery was 100% and  after two weeks it has stil over 40%. OHR was on. I never achived such battery live without this much sun sun exposure.

It was obout half a year ago. BT was disabled, no GPS activities. Only OHR, barometr and accelerometr enabled (pedometr etc.). I've never acheived such a long batery life in this mode without solar.

To have a significant decrease in battery consumption you need to stay in the sun for 3 hours with at least 50000 lux/hour, it happened to me only once and it worked during an activity with GPS