Why might acupuncture not work?

Several reasons: There are those that believe acupuncture is junk science(I'm not one of those). Others believe that you must first correctly asses the energistic imbalances before you can appropriately determine where needles need to be placed; if you can't identify the problem, just placing needles won't fix the issue. Then you must also be able to determine which points and how to stimulate them.

Acupuncture is not: likely to work if you are trying to use it to treat a condition that doesn't respond to acupuncture. Acupuncture is effective for many conditions but certainly for not all conditions.