MY question's will be a little different,
when this shifting problem is going on, by any chance is the idle very high?
and how long are you letting the atv warm up before trying to shift it?
as a very high idle could be a different issue all together!
and if the atv is in cold weather and your NOT allowing some time for things to warm up, that too can be the issue
last question will be, after the atv is HOT and been driven some, does it shift any easier?
The idle seems to be normal, it sounds the same as my buddy's 2016 Rancher 420.
And its pretty warm outside when I start it up, about 80°f and I do let it idle for about 30 seconds to a minute before taking off.
For the first 10 miles it was hard to upshift sometimes even after it was warmed up, but that seems to have gotten much better with use.
I haven't rode it much since yesterday evening, so I don't really have any new information on how it shifts just yet, but it does seem to shift easier after it warms up.
On another note, I am noticing a bit of slack in the steering, (no more than about a millimeter or two) and it's completely unnoticeable while riding, but when I'm stopped and I move the handle bars I can notice the slight play in it and it also has a slight click between moving the handle bars left and right. I have the non-power steering model, but I don't know if that makes any difference.