Most important factor for the success of IPC is:
Marginal seal over the pulp-capping procedure is of prime importance since it prevents ingress of bacteria and reinfection. Healing and the formation of secondary dentin are inherent properties of the pulp. 
Factors promoting healing are conditions of the pulp at the time of amputation, removal of irritants, and proper postoperative care such as proper sealing of the margins.
Textbook of Pedodontics Shobha Tandon Ed 2nd P: 340
Adequate seal
Material used
Patient cooperation
All of the above