Why tertiary and secondary amine do …

Secondary and tertiary amines do not for isocyanides when made to undergo carbylamine reaction. Since they have more than one alkyl group which hinders the approach of -NH₂ group.

Carbylamine reaction is a test to distinguish primary amine from secondary and tertiary amines.Now when we check the mechanism of this reaction, we find that we need to protons in the mechanism which are provided by primary amines.Seconary and tertiary amines do not give two protons , so they do not show carbylamine test.

Carbylamine test leads to formation of alkyl cyanide and this is characteristic reaction for primary amines.
Secondary and tertiary amines do not allow the formation of cyanide due to insufficient bonds for formation of triple bonds between carbon and nitrogen.
So this reaction is not given by secondary and tertiary amines.