Return of Reckoning

They generally should attach your hotbar automatically, but that is not always the case. It maybe you have potions placed there or trades...Even if space available it doesn't always take. Embarrassing to see you have not been using an important ability that was granted five levels ago.

Also, before purchasing PVP gear with crests, see if the same item can be purchased on the auction house.

The only abilities that do not automatically slot are morales and tactics, because you usually level up during combat and cannot slot them or there is no open slot left.

If you have normal abilities not being added to the hotbars, then you probably limited their slots in interface options. The hidden slots can still have abilities/items/macros + keybind on them, they are simply hidden.