Description
Cells that have NoData on any one of the following layers will be masked from the analysis and treated as a barrier: Input cost raster, Input surface raster, Input vertical raster, Input horizontal raster, and the input environment mask.
Where NoData cells are connected only by diagonal cells, the connection will be thickened with additional NoData cells increasing the masked area. If a source is located on a barrier, that source will not be processed.
Solution
To process all sources, ensure that no source is located where any of the inputs are NoData.