Crank arms moving

Hey guys, So I ended up buying a P.2 and the guy I bought from said that the non-drive side bolt kept backing out, and it does even after i cranked down a hard as I could without breaking it. It only moves about 1mm up and down, which is not a lot but still noticeable when riding. I have some blue loctite which I am gonna try. If anyone has any insight to this problem I would appreciate it. I have Hussefelt downhill cranks
Thanks!