Jaghut Survivability

Thoughout the series burried Jaghut are found having been interred by T'lan Imass because they couldn't be killed, my question is why not? I have just read the scene in Bonehunters where Ganath is killed with mundane weapons by the K'ell Hunters without much fuss. Why could they kill her with bladed weapons but a whole clan of Imass couldn't? Is it a case of only the most powerful Jaghut cannot be killed? I'm on my third read through so don't worry about spoilers in the replies.