Make sure your pins have slightly chamfered ends. Look thru the hammer hole, is much more than half of the J pin width visible inside the hammer? The pin chamfer has to be enough to ride over the curve of the J pin wire. Too much J pin showing or too little chamfer and It'll never go. You should check to make sure a part isn't just defective. Try putting the pin into the hammer only, without the receiver. It will take some light taps with a tool, like the side of a screwdriver handle. I know I can't push a pin through with my just thumbs.
Dude in video above should do the trigger pin before installing the hammer. No point trying to fight against the hammer spring, even with a punch sort of lining it up..