Why would a patient fail to respond to anticoagulant therapy with another clot?

Depends many factors: With warfarin, inadequate dosing and inadequate monitoring are probably the most commonly cited causes of treatment failure. Certain conditions such as the thrombotic disorders like protein s and c deficiency require higher doses of warfarin to be effective, "usually." it is a tough question to answer without knowing more of your medical background. See a physician.