2016 Volt had issues. Most of them small, but the worst one I recall involving loss of power steering. It was a very short model year with a run of about ten thousand units I believe. But it was the first year of the new Volt. Unfortunately, this is also the year that sites such as Consumer reports used to create their "prediction" of reliability.
CR rated the 2016 Volt reliability a 2/5. With the small sample size of 2016s and the known issues with 2016s that's harsh but not unreasonable. As far as I can tell, they haven't changed this rating for 2017 or 2018. I haven't seen nearly as many major complaints about the 2017 and 2018 Volts.
Of course actual owner reviews on Consumer Reports, Edmonds, and elsewhere are all universally positive. Probably why CR also rate the 2016 Volt's "predicted" customer satisfaction as 5/5.
As far as why Volt sales are down:
1) Sedans are unpopular now
2) Small cars are unpopular now
3) Several new PHEVs launched last year
4) The Bolt has a range that exceeds many people's "range anxiety" and it has a form factor that is more popular at the moment
5) A large portion of Volt buyers were former Prius/Toyota owners. Now they have the Prime. If the Prime went away like the Prius Plug-In then you could expect Volt and Clarity sales to rise a few hundred units each!