When Inventory.insertNewItem(short itemID, int variation, int stacksize) is used to add an item to the player inventory and the next open slot is one of the 5 quick slots, the item is not actually added (and available in the quick slot) until the inventory is not open with the I key.
Once the inventory is open, the item becomes available in the quick slot.
This issue can be tested with the Plank 'n Beams plug-in which uses this method to add the item to the player inventory.
