Additional Magical Support
We know that Barty Crouch was an extremely powerful wizard from his ability to cast a Confundus Charm strong enough to confuse the Goblet of Fire into allowing Harry to be named as a fourth contender for the Tri-Wizard cup. Given that, there is nothing that would have prohibited Barty from being able to use additional magical charms beyond simply the Polyjuice Potion which disguised his physical appearance even if not mentioned. E.g. perhaps he had a light Confundus Charm in operation around his body acting something like a cloaking device in addition to the Polyjuice Potion.
Additional Source Material Available:
Barty had access to the real Mad-Eye Moody locked up. He could have easily to ask him about his habits and practices in order to properly impersonate him. Or even torture him into revealing those habits to him. From what we see of Barty as a Defense Against the Dark Arts instructor, he was an expert on the forbidden curses both in casting and resisting. The real Mad-Eye looked pretty shabby when we first get to see him. He was clearly not being well cared for as a prisoner. Even restricting food and water can be a powerful motivator. In other examples of Polyjuice use (such a fun word play rhyme:-)) the users did not have ready access to the person(s) whom were being imitated. Barty did.
Natural Human Tendencies
Finally, we know that Barty DID make some mistakes in his actions (e.g. his tongue flicking out - at least in the movies). However, we humans (and I would think including Witches and Wizards) rely strongly on sight in identifying others. I have to believe even wizards and witches would be unlikely to constantly run around expecting someone they visually see as person A to really be person B. To do otherwise would mean someone would have to almost act in a paranoid fashion questioning everyone's identify all the time. We see several other times in canon when a person operating under Polyjuice Potion makes odd mistakes in front of others yet, nobody thinks to suspect that they are not who they appear to be. E.g. when Harry and Ron imitate Crabbe and Goyle in front of Draco in their second year. Or, when Harry, Ron and Hermione slip into the Ministry of Magic using Polyjuice Potion.
This line of thinking is supported from the Harry Potter Wikia site:
Harry Potter Wiki - PolyJuice
"The lack of suspicion in anyone in all these incidents leads one to the understanding that Polyjuice Potion is very uncommon. Granted, Severus Snape knew someone was stealing the ingredients from his cupboards and suspected Harry Potter, but that is more indicative of Snape's eagerness to blame Harry for anything than any actual guilt."
Origin of Source Material Being Imitated
Remember Moody's moniker was "Mad-Eye". He was expected to be a bit daft as our British compatriots might say. That could easily help support explaining why others didn't immediately tip to potentially odd behaviors performed by Barty while acting as Moody. Even Dumbledore.