It's too bad that Coinbase is one of the harder exchanges to work with. Coinbase consistently cancel transactions right before they are supposed to go through because they "appear to be high risk". I loved Coinbase when I first used it, but Bitinstant -> Mt Gox is far more reliable.
I've had this problem and have stopped using them. Since then I've been using localbitcoins and that's been fine. It's surprising to me that after $5 million in investment they are unable to figure out transactions.