In Finland (and several other countries), all outlets and extension cords are required to have child protection shutters. They are small plastic covers inside the outlet that are supposed to open when the pins are inserted simultaneously to both holes. You can see them as green in the image below:
On good quality outlets they usually work quite well. However especially on cheap extension cords they often fail to open.
Pushing harder or wiggling rarely helps. Inserting the plug very carefully so that the pins enter at exactly the same time sometimes work, but not always. Spraying some lubricant seems like it might help, but probably isn't very safe as it could harm the electrical contact and cause overheating.
Is there a way to force a plug into a stubborn outlet?