From the beginning I think each of the merit sources have particular board that's allocated or assign to a merit sources to merit,
Oh no - they just do it on their own terms. Merit sources are the same as you - they have habits, have advantages, have disadvantages - so they should not be forced for anything including spending their all sMerit every month. Merit source is free work - they spend a lot of time getting lots of quality posts, but sometimes they also feel lazy for their own personal reasons.
Onyeeze, don't worry - if their post really deserved an merit then they wouldn't have to post an application every 14 days in the @filippone thread. Some users just apply and take advantage of @filippone's kindness very often, but without realizing it some of them are alt who also take advantage of the same thread.