The resin curing process involves chemical chain reactions and the generation of high molecular weight cross-linked polymeric structures. The reactions concerned may involve ionic or non-ionic free radical (valence atom) mechanisms and certain chemicals may suppress the chain reaction. In your case, your additive must either inhibit or divert the chain-reaction polymerization. This result is often referred to as a quenching effect.
I suggest you use chromatography to analyse your additive and identify, isolate and then remove the offending quenching agent.