Believe me....It's not that anyone is not handy enough or skilled enough. These pre-inlet kits have this sort of thing all the time. I just bought a Chambers kit and I stressed to Mrs. Chambers that I was a scratch builer. She assured me that they have the best thing going in pre-carved. ( actually I really like the Chamber's products, except for this kit). I have only built a couple kits in my life time ( not by my choosing) and I have absolutely hated each and every one of them. It takes me twice as long to do a kit than it does to go from scratch and I have a better fit for everything with the scratch built rifle.
The trouble with kits, in my opinion, is that the kit maker wants to make it soooooo easy that they go overboard and loose track of the process of hand inletting. When I inlet a lock I know exactly where I want it and how I want it because I have taken the time to figure it out and map it out in my mind. With pre-inlet you have to go with the opening already established, but since it doesn't quite fit, do you take off at the front or the back, the top or the bottom, will everything work right regarless of which end recieves more wood removal even if the external apprearance is good????? Too much second guessing.
I don't mind having the butt of the piece pre shaped and the barrel channel and the ram rod work pre-done, but that's where it ends. Everything else I prefer to do myself, from scratch.
This kit I have now is giving me fits, just like your problems, and I have built many rifles in my time. No lack of experience here. Good luck and belive me when I tell you, "You are not alone"!.