There is a number of opportunities to apply for higher positions.
Because it is a smaller agency and people generally like their jobs and perform at a high level, progression seems relatively slow. There is a lot of competition for any job.
At entry and mid-levels, it is fairly easy to be promoted and there is a formal process to follow to make these opportunities happen. At the higher levels, there is a bottle neck and it is more difficult.
It is not that easy as budgetary constraints limit the amount of positions offered at higher classifications.