It's only as simple as selecting "pack" a second time if you are on flat paved ground... On the dirt it often gets stuck halfway.
What I do is unpack it, winch the back of it to a tree or something and pull it out to extend it by driving the truck forward. Then I bring up the advanced menu again, and quickly select "pack" twice. This will try to fold it up again with the first press, but before it gets to move much it tries to snap back out to unpacked form, and sometimes it clicks in place.
Even doing that it only works about 50% of the time though, sometimes it just refuses to lock into the unpacked state. You can still load logs in it and choose "pack logs" and it will straighten out when the logs are packed in.
I'm on console though, if that makes any difference.