5 months is really early to expect to lose it all. I remember, particularly with my first, hitting plateaus like that. First lost 17 pounds then it seemed to take forever to lose the next chunk. I'm sure it was close to a year before I got all the baby weight off. My sister has had the same experience.
With my 2nd, I started Weight Watchers when he was about 6 or 7 months old. It worked like a charm. Since I was breastfeeding, I got to eat much more than I otherwise would. I easily dropped 2 pounds a week and not only lost all the baby weight, but an extra 20. However, I noticed my milk production was significantly lower the last many months with the my 2nd child. I'm pretty convinced it was because I was actively losing weight and then had lost so much weight. Fortunately, I always produced way more than needed so this wasn't much of an issue, but you should think about that. You don't want to jeopardize your child's milk supply.
Another thought - and this is strictly my anecdotal observation. All the people I know who've had a lot of trouble losing all the baby weight are people who were pretty skinny to begin with. The people I see who lost it more consistently started out at a more average to above average weight. I have read this - you need some fat to generate breastmilk. I think if you are thin to start with, retaining some of the weight is your body's natural way to help you keep producing the milk.
Last thought... I was always hungry when I was breastfeeding (a year at least with both) and I realized that I was eating a lot as a result. This made it harder to take the weight off. So pay attention to your diet and make sure you are eating plenty of healthy foods.
I know it is really frustrating! I felt the same way. But with time and some work, the weight will come off.
Good luck!
C.