Acupuncture is ineffective if quacks are the ones who do it without any understanding of what the damn hell is actually going on.
The way I interpret it, acupuncture causes micro damage, and the body then says 'whoa there's damage over there, I'd better do something to go heal that spot'. Same as when you lift weights, get micro tears in your muscles and then the body rebuilds the muscle fibres and makes them stronger.
My mom had swelling and water retention in her right leg for the longest time. Her body had settled into a holding pattern of 'if it ain't broke, no need to fix it'. I urged her to go for acupuncture at a local hospital specialising in traditional chinese medicine (with certified doctors), and after 2 months, the leg started to heal.
All I can say is that it takes a while for the effects to show IF a trained professional is doing their job right.
If you've had a bad experience, that doesn't necessarily mean that acupuncture doesn't work. In many cases, a lot of quacks are masquerading as the real deal because it's easy to hoodwink people when there's mystical oriental stuff involved.