Some manufacturers have changed the Line side of their GFI receptacles. More than once I have had an electrician not realize and just wire the outlet in his “old” way. (Old way was Line in top: Protected Load in bottom. With the bottom screws taped over by the manufacturer to warn they were load protected. Some newer GFI receptacles have reversed this, the LINE gets wired to the bottom screws, and they stopped putting the yellow warning tape on the protected load screws). This leads to a condition where, when tripped one side of the outlet will no longer have power but the other side will. It can also show up as crossed wiring when tripped on down stream outlets.
The best way to test, is plug the tester in and trip the GFI. Then BEFORE resetting, plug the tester in to the other outlet on the GFI receptacle.
The not resetting is a whole other can of worms. There is a line of GFI receptacles out there, that are about 7-12 years old. They never had a reason to ever trip nor be tested. They live test fine and are wired correctly. But once you trip them, they wont reset. The receptacles themselves are bad and were poorly made at the time.
Listed GFCI receptacles that are wired incorrectly cannot be energized. It has been part of the UL listed for a long time that if you reverse the line and load connections you cannot energize the receptacle. If it’s an unlisted or counterfeit Chinese brand well than anything is possible. Travis is correct that the yellow label that covered the load terminals is no longer required.
These were actually Leviton GFI’s. Electrician miswired 3 in 3 different condos of a highrise I did work in. All 3 worked fine untripped. When tripped one side stayed hot the other side went cold. The note here is that the miswiring did cause all 3 to trip. They could then be reset, and would not trip until something was plugged into that specific outlet of the receptacle. But it still left one outlet hot.
