KNO₃, NaNO₃, LiNO₃ and RbNO₃ are hygroscopic, AgNO₃ is non hygroscopic.
The combustion reaction of hexamine is (CH₂)₆N₄ + 10O₂ → 4CO + 2CO₂ + 2NO + 2NO₂ + 6H₂O

Therefore a chain reaction starts.

The more H₂O the alkali-metal nitrates absorb, the more energy they need to release O₂. On the other hand hexamine is decomposed as following:

(CH₂)₆N₄ + 6H₂O → CH₂O + 4 NH₃

The ammonia solution is alkaline because of the reaction:

NH₃ + H₂O ↔ NH_4⁺+OH^-

Adding copper sulfate solution to the aqueous reaction product of hexamine and the alkali-metal nitrate, leads probably because of the residue of hexamine and the decomposing of water and hexamine to copper(II) hydroxide:

[Cu(H₂O₆)]²⁺+2 OH^- → 6 H₂O + Cu(OH)₂

Hence, my new theory is, that hexamine can only be oxidized with non hygroscopic nitrates.

Author: M. Bindhammer, Team Selene