Augment active skills doesn't stack with normal buffs!!

Hi! I am playing at an Interlude Lineage 2 server and my problem is that some augment skills doesn't stack with normal buffs. For example, i got an "Empower" augment active skill with my Storm Screamer and it doesn't stuck with the "Greater Empower" buff from the npc buffer (In fact the Greater Empower is much stronger so the augment "Empower" doesn't even appear when i cast it having "Greater Empower"). So my question is...is that normal to happen? And if it is, then what is the point of having active augment skills if they don't stack with the similar buffs? Thank you!