Hydrolysis equilibrium of urotropin
Small amount of acid is required for hydrolysis.
As a result ammonium salt and formaldehyde are produced.

Update:
It can be explained with enthalpy of reaction Wiki Enthalpy of reaction. 
To calculate it, enthalpy of each  reactant required. 
From wiki: ΔH(C6H12N4)=-99.2kJ/mol, ΔH(H2O)=-285.8kJ/mol, ΔH(NH3)=-45.94kJ/mol, ΔH(CH2O)=-115.9kJ/mol; 
ΔH(reaction)=6*ΔH(CH2O)+4*ΔH(NH3)-ΔH(C6H12N4)-6*ΔH(H2O)=918kJ/mol; 

As ΔH(reaction) is large, positive number, a lot of heat required for this reaction, as a result, equilibrium is far on the side of the reactants.