I paid 16k for Dependent Care but only $2560 is coming up as the tax break. Why? I need to write off the entire amount.

The dependent care credit is a percentage of the expenses, not the full amount. The percentage is a sliding scale depending on your income. Form 2441 shows how the credit is calculated.

Also, if you had dependent care benefits from your job, that reduces the amount of the credit that you can get on your tax return.

As @rjs states dependent care expenses lead to a credit that is a percentage of those expenses and are not a deduction from income. 

The difference being a credit is dollar for dollar off your tax bill whereas a deduction simply reduces your income and thus only reduces your tax bill according to your tax bracket.