Is the bolt head extended and flat to the reciever? You may have knoticed when it was removed, the bolt head stuck out from the bolt body, now it has retreated into the bolt. Pull it back out, it will rotate 90 degrees. It should rest flat against the reciever, and will take some effort to keep it from snapping back while guiding the bolt body grooves onto the reciever. Don't force it as if it isn't lined up you'll just be mashing away at the parts.