iPhone 4S stops charging via USB when I get message alert

Hi.

My works iPhone 4S stops charging when I get a message or e-mail alert or answer a call. On site I generally leave it plugged into my laptop as the battery life is pathetic. My own Xperia has never stopped charging even when in use. Is this an Apple trait I have to get used to (never owned or used anything Apple before) or is there a problem?