Board Thread:Speculation House/@comment-66.87.78.150-20151129211046/@comment-11188061-20151130012135

Fact: The Schnee family summons are Aura materialized in physical form.

Speculation: Weiss have problems summoning the Knight (or the sword it wields) because there's a high possibility the Knights is related to her father. It may even be her father's summon. She refuses to have anything to do with her father, so the thought of summoning the same creature as him galls her and affects her ability to perform summons.