Maybe is it because there is a known defect in the phone's design that causes the charging port to fail even in normal use. Look at the hundreds of complaints about customer not being able to charge their phones, like the Ace and Blackjack, and Samsung's refusal to make repairs under warranty because of their poor design.
It's the battery port. I'd suggest to either buy a new battery, or a new phone altogether. Don't get me wrong, I love my blackjack, but dealing with it not taking a charge can be very frustrating, let alone inconvenient. Samsung, to my knowledge isn't doing much to fix the failed battery ports found in most samsung blackjacks, so if you, like myself, really do like your phone, I'd suggest buying an extra battery, and keep them both charged..when possible.
Maybe something is wrong with the battery or its the charger it self. So you can go check on both those things and see what's wrong
Battery is empty so I recharge it but when I plug it to the power I know that the mobile is not on like before why?