They limit their Dwolla exposure at any given time so Dwolla transactions are often back-logged.  There are likely other people ahead of you in the queue for Dwolla processing so your request may not get processed until a previous batch of withdrawals has fully cleared.

I sometimes think MtGox would be better off dumping Dwolla altogether as their risk management strategy for limiting potential losses is extremely customer-unfriendly even though it's very necessary.