I wasn't trying to lose weight when I went paleo, but I did, effortlessly and immediately (which is odd for me--pre-paleo, I had to watch my calories like a hawk to maintain my weight, which was incredibly stressful and depressing). I was about 115 lbs (I'm 5' 3") when I started, and was down to 105 in a month (it's worth noting that I completely stopped counting calories). If you're on paleo and not losing, my guess is that your diet is too fruit/starch-heavy. Try trading in things like fruit, squash, and sweet potatoes for high-fiber, low-sugar/starch veggies for a while and see what happens.