If this is a 3" model (a distinct possibility since they seem to be making a lot of those lately), check to make sure that your fingers on the back of the knife are entirely off of the lockbar - effectively, keep them on the clip while you open the knife.
I ran into a similar situation with my 3" Slicer, purchased a few months back, which felt at first like the detent ball was more like a pin dropping into the detent and locking the blade closed. Combined with the smaller thinner end of the flipper, it was biting into my finger a bunch. I'd never had this problem with my 3.5" Spanto, but that pits roughly the same detent mechanism against a much longer lockbar, and a much heavier blade. After a drop or two of oil into the detent, and working the action a bunch while training myself to not put any finger pressure against the lockbar (holding it closed) while opening, it now opens quite quickly with no finger irritation whatsoever, and has become my favorite of late, getting carried everywhere.
I don't mean to downplay falcon's advice, BTW - Rob is very responsive and RHK has awesome customer service - by all means, consider chatting with them and possibly sending it in. It's just that, if this tip clears up the problem, then you won't have to wait, empty handed and forlorn, while your shiny new XM-18 makes a round trip to The Ranch.
Also, FWIW, I never use the thumbstuds on my Hinderers - flipper all the way. It was a definite change from my previous knives, now I can't go back.