When will the Lassaigne's method for detection of nitrogen and Kjeldahl's method for quantitative estimation of nitrogen fail?
You are right about the limitations of century old Lassaigne's method. Why are they still teaching it? Modern day CHNSO elemental analysis is done with gas chromatograpy systems. If you are quoting from a book, provide a reference.

This is an excerpt from a 1953 work Sulfur in Organic Compounds, by a Modified. Lassaigne's Method ACTA CHEMICA SCANDINAVICA 7 (1953) 1395


  Objections to Lassaigne's method are: a) low boiling substances evaporate
  before reaction with the sodium, b) diazo-, azo and polynitro compounds evolve nitrogen c) some heterocyclic compounds do not react.