I suspect there's a 'prove yourself' period with STN. Maybe also an internal rating system. We request shops, and overnight, their site assigns - or not. If you don't get the assignment, but still see it on the board, you request it from the scheduler. Then, another day or two until you know if you got the shop. Argh. A friend of mine, newer to shopping, is assigned far fewer shops than I am.
They post their shops the end and first of the month. I am already working on routes with other MSPs, that fit into my schedule. Self-assign rights with STN would be sweet and make life a little bit easier.