It is likely a connection issue somewhere in either the plug, circuit, or possibly an incorrectly wired circuit. Sometimes DIY folks can get the meaning of the grounding conductor and grounded conductor confused. If those were swapped somewhere in the circuit, the return path would likely be going through earth. Depending on what is plugged in, an incandescent lamp may seem OK but a piece of electronic equipment might just balk at it. I think one of those "u ground" testers with the 3 lights on it should tell immediately if there is an error in the circuit.