I am playing WHM and I noticed that oftenly if you try to reapply Regen on your tank on the last second timer it results in No effect message and the buff is not applied to target.

I can't say if it is something with specific additional buffs on target or lag but I have met this behaviour multiple times. I hav espent both mana and GCD in vain.

This may however have something with WAR being a tank cause I can't recall these problems be there with PLD being a tank. I remember exactly being able to 100% reproduce this bug on purpose.