Finally got all 4 GPS receivers to lock, but still, I find that not all of them keep the lock. Turns out, theoretically identical receivers (e.g. bought at the same time from Parallax) aren't.. There's two different antenna styles, and obviously, they work differently. When tested side by side in my precision outdoor test range (see attached picture) I found that the ones where the patch on the top comes closer to the edge work worse than the ones with the smaller patch. Fascinating. Not surprising once I identified the difference. Now I'll have to see what the C/No reported by each one is.