The blank bale drills by Rod Jenkins in the DVD Masters of the Bare Bow III should give you exactly what you are looking for.
The reason you can't diagnose why you missed is because there are probably multiple reasons, some of which you aren't aware of. As you progress, and develop your form, you will get to the point that when you miss, you will know that you missed for some particular reason, and it is rewarding in itself to be able to correct something and have the next shot hit your mark. But that doesn't come overnight, or easily.
In the mean time, there is no reason you can't just enjoy your shooting, and appreciate the occasional great shots you will make.