My KitchenAid mixer head will not lock in the down position. How can I rectify this?

Lift the head all the way up. On the top of the stand, just next to the mixer itself you will see a little square hole. Check if that hole is obstructed.

If it looks clear, then check that the set screw on the right side of the stand is not missing (as suggested here)

This is typically caused by the head of the mixer being too high above the bowl.

KitchenAid suggests using a dime to verify the bowl and mixer are the proper separation:

http://producthelp.kitchenaid.com/Countertop_Appliances/Stand_Mixers/Stand_Mixer_Assembly_and_Operation/Lock_lever_on_tilt_head_stand_mixer_does_not_lock%2F%2Fmixer_head_moving

There are two things to consider:

One, the mixer is too high above the bowl. This will result in unmixed ingredients and a lock that will not engage.

Two, the mixer is too low. This will result in increased wear on the mixer blades and the mixer itself as the head and bowl are pushed around during mixing.

When the mixer is lifted a screw should be visible inside the hinge between the base and the mixer head. Turning this screw (with a flat-head screw driver) to the right (clockwise) lowers the head and beater, and to the left (counter-clockwise) raises the beater.

KitchenAid recommends placing a dime in the bowl because it is about 1/16th of an inch thick and will help you gauge the correct height of the mixer head.

Personally, I've found there seems to be some distances where the lock engages and some where it doesn't as reliably, and that these zones don't strictly correlate with the height of the mixer itself. But by trial and error you should find a point where there is acceptable separation between the mixer and bowl, and the lock is able to engage properly.

I was able to correct mine by adjusting the flathead screw that the head rests on. If you (carefully) flip the whole thing upside down, then try to lock the head, you’ll see the little metal “arm” try to push through the hole, which in my case was misaligned. I was able to align it by screwing in that flathead screw a little bit.