Not all transactions can be withdrawn as they are used to offset debits such as the DirectCam, Direct Messenger and Tips Facility Fees (what's this?).
For example, if you carry out any sessions ( cam, direct messenger, etc) that have a facility fee that will be debited at the end of the session, say for 10 credits and you are then charged 0.75 credits for the facility fee, you will not be able to withdraw that full 10-credit session, as it would result in us paying you 0.75 credits more than what you are owed.
This doesn't mean that you can't withdraw all sessions, just that occasionally some get marked as 'not transferable' to stop the transfer page showing items that would result in us overpaying you.