[Sever]

My current character is a thief in the purest sense of the word-- I sneak, spell or sleeze my way past obsticles. But i'm having some trouble with this spell. Whenever i usually cast invisibility, it's 100% effective. But i'm at a point in the main quest where there is a certain burial chambers filled with skeletons who keep shooting me whilst i'm invisible. And the bastards are hitting me every time! How is this possible? I don't recall any spells of 'detect invisibility' in the game. Even when coupled with chameleon (40-70%) and sneak they can still shoot me. Any ideas on what i'm doing wrong?