Just because weight has come off quickly in the past doesn't mean it's always going to come off fast - as i've learnt this time round.
1.5lb is the average weekly loss on SW, so don't be hard on yourself, you're doing great! If you carry on at that rate then in a month you'll be in the 13's.
I know it's easier said than done, but try not to compare yourself to other people too much, we're all so different. Larger people are often more likely to lose weight quicker, as are more active people, our metabolisms are different etc etc etc - too many variables to compare against other people!
Also, it's worth noting that in every class there are always people who have big losses for a good few weeks, who lose a good chunk in a month or two, but then they disappear, only to come back to class having to start at the beginning as they've put the weight back on again - it's the slow and steady losers that reach target and stay there!