As a connection gets warm it will oxidize. Resistors are made out of metal oxides. As power continues to flow through the resistive material it only gets hotter and eventually compass or melts the receptacle. Heavy duty receptacles have greater holding force, more points of contact and higher temperature plastics/ceramics. A stronger holding force can reduce the amount of oxygen reaching the junction mitigating one factor. More points of contact distributes the flow over wider surface reducing the initial temperature increase. And the other factor just allows the receptacle to function much longer through the degradation. There is one Super cheap option that might just allow you to continue without upgrades, dielectric grease. The only purpose of dielectric grease is the cover surfaces and prevent oxygen interaction and oxide formation.
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