I purchased a White Winter Owl. I leveled "Ginger" up to a teen and she got guardian spirit. The wiki says that the trigger to the talent is when attacked or successfully casting. I tested out the talent on a lost soul, and sure enough, it attacked, but my pet did not cast tower shield on me. It may be me misunderstanding, BUT if this is a glitch or a bug, please fix it.
Do you mean that your owl didn't cast when a trigger occurred? Pets don't always cast their abilities, even when major hits are played. Just keep battling and questing, and your owl might cast it along the way. Hope that's what you meant!
Mackenzie Stormblade