Wraithlords and Wraithguard alike suffer from a phenomena called Wraithsight, which describes the temporary inactivity of these wraithbone-constructs.
The Wraithguard and Wraithlord "bodies" are activated by the implantation of a spirit stone containing the soul of a deceased Eldar. Due to the spiritual nature of the piloting souls, the physical world appears to them as an ever-shifting melange of shapes and colors. This makes the wraith constructs slower to react to changes and stimuli, and often results in the construct becoming unreponsive for a period of time.
Knowing this, the Eldar often send a Farseer or Warlock with these constructs, as the guidance of a psychically potent mind is very effective at combating the effects of wraithsight. Warlocks that specialize in this are called Spiritseers, and are most common among the ranks of the craftsworld Iyanden.