I'm going to guess that the issue is too fast/not enough heat.
It turns into a matte like color once it's heated enough. During the course the instructor would come and look at all angles... any shine/gloss he saw he'd deduct points. So I'd take a look and make sure it's turned matte.
I haven't done any personally, but we do have some up that have been up for 5+ years using IJ 8624, with no tenting. We don't have the harshest winters/summers though... So that may play a role in it.
I do know you have to pretty much do it perfectly or it wont last long though.
Even this guy is going a bit faster than we were taught.
Make sure the grooves are in the middle of the roller - Even if you have to overlap... that ensures the best pressure on the center. Also kind of do a "Wiggle" when going over verticle grooves to make sure you're pressing them in, and not just skipping over it.
For the 3M preffered course... the brick wall is the one most people fail on (Or so I'm told) Because it's tricky to do perfectly... and anything less than perfect you'll get an early failure.