Headlight
Re: Headlight
It is either not being locked properly in place or the hardware that is twisted to lock it in place is broken in some way. the much more common problem is that you can't get the lamp assembly put to change the bulb.
Re: Headlight
Its tricky to get those bayonets into the right locking place - if you don't feel tension increasing on them as you tighten them, you don't have it right.