A compound is deemed "soluble" in HCl if it is significantly more soluble than in water.
Electronic and steric effects of substituent groups will affect basicity, and, therefore, solubility
in 5% HCl solution.
Some hydrochloride salts of amines are insoluble in 5% HCl, therefore, some amines will appear to
be insoluble. Warming the mixture slightly will dissolve these salts.