There are a few reasons a job board may not be available for selection on the Sourcing Page.
1) The job already has active sourcing to that job board. Once a media has been selected for a job, that sites option is no longer clickable on the Sourcing Page. The job board will become available again when the active sourcing has expired.
2) A posting contract has expired or depleted of inventory. If you have a posting contract / subscription with a job board and that board is no longer available for selection, it may be that the contract has either been depleted of inventory or past the expiration date associated to the contract.
3) The job may not fit the sites posting criteria. Sometimes a board can be grayed out if the posting does not fit a key element of a sites posting criteria. For example, if a job is located in Europe and the board only accepts USA based jobs.