Sounds like a knife line marking problem. I assume you cut your tails on one board, hold that board against your pin board and then scribe around each tail to mark out the slots on the pin board. If I follow, you are saying that when you cut out the slots on your pin board for the half blind dovetails, you have to cut a hair shy of the scribe line. If you cut right to the scribe lines, your joint is too loose. If that is what is going on, it means you are not getting your scribe line tight enough to the tail. Teh question is why. Paul sometimes talks about marking things as if you were trying to reach under the piece of wood (here the tail) with your knife tip. Actually, I suppose you do get the slightest bit under, though just a fraction of a fraction of a fraction of a mil! It is so little that it is almost like imagining it going under. If you go too far, the joint will be too tight of course. But if your knife feels like you are going straight down (that is, not the slightest almost imaginary bit under) you will probably be too loose. So my guess is that your knife blade is too vertical rather than sort of reaching that slight almost imaginary bit under the tail.

But keep two related things in mind. First the bevel of your knife can get in the way, It can really throw a joint off if you let it. You need to get the tip of your knife in there without letting the bevel of the knife push the tip out. Second, even if the bevel is not a problem, the knife blade itself has width. The deeper you cut when marking a line, the wider the line your knife makes. If you have sharpened your Stanley knife, you may have even changed the angle and hence the fatness of the line. I have done that myself.

One thing I do sometimes, rather than have a difficult to see knife line on end grain, is instead of using a knife, I use a B or 2B pencil, and shade in to the corner. That gives me a nice clear delineation of what is to be kept and what not (which can save on hair pulling). And any overspill on to the tails also helps to highlight any tightness that might need paring. Cristian Becksvoort demonstrates this technique on his video for FWW (Just another dovetail video, I think it is called).