Our new home came with a set of electrical outlets which have some strange behavior. The new building code requires them to have built in child proof protection.
Some of the plugs I have go in easily, others are so difficult that my wife can't get them in; and I barely can.

I obtained a new outlet and split it open to see if I could figure out the why. The best I can tell, without a micrometer, is that the newest plugs (that are hard to put in) are a bit thicker and stiffer than the old ones (which are generally easier). When the plug is inserted, the little plastic piece you see attached to the spring slides sideways.
A real puzzle. And I thought that Paul might have the answer.