If I remember correctly LB are group based on roles:
1. Tank
2. Melee DPS
3. Ranged DPS
4. Magic DPS
5. Healer

They are all different mechanic.
For Melee DPS is as simple as click on target and cast LB. For Ranged I think we need to assign the direction. Magic DPS is AOE (probably red circle). Healer/DPS is cast right away.

The probable reason of not being able to cast might be this, or it's get interrupted.