Sometimes credits that you have earned, cannot be withdrawn as their value is used to offset debits that have been made to your account.
These transactions will have been offset against the DirectCam and/or DirectChat facility fees.
Untick the "Hide processed credits" box and you will see that what you have earned versus what you have been charged/spent adds up to the amount available for withdrawal.
More information on how transactions can get left on your statement and not be withdrawable can be found here.