Line of sight uses origin points, and your walls are blocking each other, for example:
This will be difficult to fix. You'll probably have to make collision polygons slightly smaller and add invisible "detector" sprites to each corner of each wall block. And then check LOS to those detectors. If at least one of them is in sight, then the whole block is considered as visible.
Also, I strongly suggest scaling everything up, say, use 32x32 px blocks. Having 1x1 px objects is super inconvenient and applies many limitations.