We had a new starter in our group 2 weeks - she lost a stone in 1 week! Even the consultant was surprised. This week she put on 2lbs.
Think it depends on how much weight you have to lose, and how different slimming world food is to your regular food. If you were eating healthy in the first place then you should not expect massive losses initially, regardless of how much you have to lose.
Also - you mentioned you have a run of 3lb losses followed by a maintain - is the maintain on * week? As that is not unexpected.
Finally - don't give up!! I am 15 weeks in & have lost 1.5 stone. I average 1.5Lbs a week loss, am happy with that and if it carries on at an average of 1lb a week I will be at target sometime in May next year. I am happy with that as it means I'm likely to stick to it long term. It will be the best £300 I've enver spent.