Melting Maelstrom - not recovering all mana when used

I'm playing a Lightning Arrow Deadeye Ranger build and trying to use the Melting Maelstrom flask. However, I'm encountering an issue where it doesn't recover all my mana when used. Instead, I get the message "I'm out of mana". This happens no matter which skill I'm using to deplete my mana, when my mana is fully drained, the flask doesn't restore it. Am I missing something?

Solved: It has to be manually triggered :))