I'm building my first wheel, and I've already dismantled and relaced it once because of this problem. It's much better now, but I'm still struggling. First off, let me just say that I'm all too aware (and becoming more aware all the time), that a 4x 20" 48 spoke wheel on deep section rims was not the easiest thing I could've chosen for my first build, but I'm an idiot, and here we are anyway.
Essentially, I'm now installing the leading spokes, and I've reached a point where whilst the spoke still reaches the nipple, a combination of the break angle and the fact that not all the nipples are completely seated means that whilst the spokes will reach the nipples, I can't get the threads to engage. Sheldon says "make sure the leading spoke nipples are fully seated". Well, I've been trying to do that all the way along, but not quite succeeded, and now I don't know how to. I've tried to do it by manipulating the hub, and that did enough for a while, but I've reached a point where I can't get any further. Can anyone suggest what to do next? You can see the problem in the pic below, I think:
It's not altogether clear, but the trailing spokes you can see, where the whole of the nipple is exposed, aren't actually engaged, whilst on the leading spoke in between, the nipple is raised into the rim cavity.