I suspect you are talking about a PA28. If so, then you can preflight as much as you like, these things do pop open occasionally, latched or not. Happened to me a couple of times during my training (I learned to fly on PA28s, now avoid them if I can). No big deal, basically just keep going. Absent an autopilot it's probably much more dangerous to try to close/lock the door than to simply let it trail slightly ajar. From distant memory the POH suggests to open the storm window on the pilot side before attempting to close it.