sometime organic compounds both nitrogen and sulphur cant form blood red coloration in detection of nitrogen why?

Lassaigne’s solution is aq. Ferric Chloride (FeCl₃) that reacts with Sodium Thiocyanate (NaCNS) to form Ferric Thiocyanate [Fe(CNS)₃] which is blood red in color.

This test can be used as a confirmatory test for organic compounds containing both Nitrogen and Sulfur (e.g. Methionine amino acid). Fusion with metallic sodium followed by aqueous extraction and reaction of the solution with aq. Ferric Chloride yields a blood red color for positive response.