This is a more general pattern in theoretical research. When you first start to attack a hard problem you often notice many promising lines of attack. Somehow in every line of attack, there will be at least (and often exactly) one thing that doesn't quite work out. Terence Tao has described this as feeling like "enemy movements" or something like this (though I can't find the quote). It is generally not possible to cross such gaps until you actually understand them. Once you do, instead of looking for a path from premises to conclusions you look for any gap in the chasm that seperates them. Once you've found the gap, it's often easy to go from premises to the gap and then from the gap to your conclusions.