Unable to set Singularity grid total to 16
Make sure that you aren't overwriting your grids variable.  I believe that by default, it's overwritten when the browser reaches the $tab size via: $grids: add-grid(12 at $tab); so searching for that line would be a start.

To help visualize your grid, singularitygs has the @include background-grid($color: blue); option that might be helpful while testing.

Lastly, you can always set your grids on the fly with a 3rd argument in your grid-span calls.  For example:

// Override the default $grids variable to use 16 grids
@include grid-span(16, 1, 16);