It is easier if you first remove the bolt hold open lever. Also watch the position of the firing pin tang that sticks out from the back of the bolt, it has to fit in it's slot first.
This is one of those things thats hard to describe but easy to do. The tang has to go in that slot with the bolt tilted and kinda rotated a bit as it goes in. It does go in with no effort or drag but has to be done just so.