Recently I haven't lost weight because I've continued to eat junk. I eat too much, and I eat the wrong things. I get such sugar cravings, and even this week I'm having trouble not giving in to them (read, I gave in to them). I don't really know how to get rid of those cravings without going on phase one (South Beach), and you're not supposed to do that when you're breastfeeding.
I also haven't lost any weight because I haven't exercised. This is partly due to having a baby who needs to be feed, changed and generally looked after, and I'm better at sitting in front of the TV feeding her than I am getting up and doing something. Like going for a walk - I'm hopeless at going for a walk without a proper purpose (like running an errand, going to the doctors, etc).
My treadmill is at my mum's house, and though that's only 20 minutes away, it's 20 minutes I can't always be bothered to drive given the choice (all the baby stuff that needs to be taken, timing it with feeds, still taking 2 hours to get out the house, the need to get out of my pyjamas before 3pm etc etc). And then the times we do make it over there, I haven't got on the treadmill until today.
I'm sure there are lots of other reasons. I'm hoping that as I begin to identify them I can begin to make changes that will result in weight loss :)