Image: WowheadIn World of Warcraft: Forever, players must purchase new and upgraded ranks of their abilities from their class trainers. When you purchase a new spell rank as a mana user, though, or as a melee class that uses mana-based abilities, it may not automatically update to the highest rank you have on your action bars.
Reminder: update your spell ranks Melee players like Rogues and Warriors won't need to worry about accidental downranking of their abilities, since their action bars will update automatically when a higher-ranked ability is learned. Other classes, however, have had the option of downranking their spells for lower costs at the expense of damage.
Double-check and make sure your spell ranks are correct. Just because you purchased it does not mean it's on your action bars already.
The default UI for your Spellbook will not show lower ranks. To see lower ranks, hover over the down arrow at the top right and then select the option to show all ranks of spells.
Source: Wowhead ↗