I'd like to echo Andrew's suggestion of testing the traces from end to end.
It is tedious, but testing each trace that runs under or near where the runners sat may find your problem.? A signal trace will usually surface somewhere on a via, and then go back under again after traveling some distance.? Use a set of fine and extremely sharp test probes to test the continuity.? Jam the probes into the end vias at an angle to pierce the solder mask so that it makes contact with the plating in the via hole.? Don't trust a trace is ok by visual inspection alone.
I have a set of Pomona 6275 with SS tips that I keep sharp with a hone for this task.? You could also make your own sharp probes from sewing needles if these are too expensive for your budget.
Note that most of the larger solder blobs are not vias at all (unless they have a hole in them), but are test pads.? As he also points out, it's a very common spot for corrosion attack since the bare copper is usually exposed around the edge of the solder blob.? You'll also notice that many of the solder blobs just go to a via and nowhere else.? Those are signal test points and not worth worrying about.? The solder blobs where the trace passes through it are the ones to make sure are incorporated in your end-to-end testing.
-mark
