Is the conjugate base of every polyprotic acid amphoteric? Since all compounds with an accessable free lone pair can act as a Brønsted-Lowry base, I was wondering if there was any specific example of a substance that when it loses a proton, it cannot accept a proton but can donate another proton. 

Please note that I am not considering something like sulfuric acid which is a strong diprotic acid, as $\ce{HSO^{-}_{4}}$ still accepts a proton even if its to a limited extent.
The conjugate base of sulfuric acid, bisulfate ion, shows little amphoteric character in water solution.  Unless you drop the pH to about 2 or below, it is only an acid, and a moderately strong one at that.