Basically, the system plays items according to the following rules:

Paid Content always plays before unpaid content regardless of priority.  There are three categories of content in the Paid Content tier; Paid Spot (CPM), Paid Spot (PI), and Target Spot.  There is a fourth "level" of items that falls after the Target Spot category and this is the Ad Wire level.   

If no Paid Spots in any of the three categories, or Ad Wire, are available to be played, the system looks for items of Higher Priority and it chooses the highest priority item available for play and plays it.  

If certain items are not playing, this is usually due to the fact that there are other, higher priority items that are being pulled and played.  To get "non-spinning" items to start garnering plays, look first to equalize all the priorities of all items in the unpaid categories (Unpaid Spots, Promo, Song, Bumper, Sweeper, Short Show, etc.  Anything other than the paid categories listed above).  If certain items need to play before other items the system will determine what gets played based on the Priority of the item.

We hope that helps.  Please let me know if you have any questions or need any further assistance.