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To charge the iPad or most any tablet it requires a 2A output. Most USB ports only output 1A, that it why it would charge you phone just fine but not charge the iPad, that is also why the charger for the iPad is larger that the one for the iPhone.
The USB port most likely will only output 1 amp to charge an iPad it requires 2 amps. Just like trying to charge it on your computer. It should say the same thing unless you have high output USB ports.