What actually happened? My understanding was that politicians back home refused to give Hannibal the full support he needed until it was too late.
*braces self for Dachspwn*
There were several reasons why he didn't get enough reinforcements, lack of support from Carthage was not one of them. In fact, the Barca's most likely had a lot of support, and Hanno's group were a minority, only placing the blame on Hannibal starting the war when he wasn't there and the Romans were in their backyard.
The Battle of Dertosa in 215 BC won by the Scipio brothers against Hasdrubal Barca's army he was bringing to Italy reveals some of their reinforcement plans from both Iberia and Africa. It really dealt Hannibal's efforts a terrible blow. Not only did he lose a large contingent of men coming to support him from Spain, but massive reinforcements from Africa were sent to replace those men lost to the Scipios to defend Spain.
Essentially, this victory prevented Carthage using up to four armies in Italy. Hasdrubal in the north would have applied pressure on the allies there, while Hannibal would have had new troops to defend his allies better while allowing him to take back the initiative lost due to not being able to be in two or more places at once, while Spain would have been fairly secure as the Romans may well have been concerned with the large Carthaginian forces in Italy.
At such a critical time, with Rome on the back foot after Cannae, this could well have been what Hannibal needed to win the war... but of course, some things were not in Hannibal's control.
There is only one recorded instance of Hannibal receiving reinforcements from Carthage, which arrived at Locri on the Bruttium coast in 215 BC under the command of Admiral Bomilcar - they had transported a force which included 40 elephants, cavalry and infantry. They also sent armies to Sardinia and Sicily. Sadly, without Hannibal leading the Sardinian expedition, the army was defeated, while those sent to Sicily ultimately suffered an epidemic which wiped them out. As late as 205 BC, Mago was sent with lots of money and troops to Northern Italy where he hired mercenaries and recruited, captured Genoa and held an area of Northern Italy for a couple of years, where he also received reinforcements from Africa.
Though he did rely on support from Carthage, his main hope, I believe, would have been support in Italy. When speaking to Antiochus of Syria on invading Italy, he said that 'Italy would provide both supplies and troops to an external enemy.'
