Not all transactions can be withdrawn as they are used to offset debits such as the DirectCam Facility Fee (what's this?).
For example, if you do a session for 10 credits and are charged 0.75 credits for the facility fee, you will not be able to withdraw that 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.