Hiya, sorry to hear you're having a hard time. HOWEVER... a few points to consider:
1, losses can slow down that's true, but are you actually trying 'too hard' by not eating enough? It will make your body shut down and hang on to everything it's got. When doing dukan it really seems to be that the more (protein) you eat the better you lose. Are you / can you post up menus with rough quantities then we might be able to see if there is something wrong? If you are following it to the letter, including eating enough protein (2g per kilo of bodyweight) then you ought to be losing ~2lb per week (up to a point, see below). I think you are actually losing at that rate, apart from the stall in the middle? So you will get to target a bit later. Anyway you lost 1 1/2 stone in 6 weeks you can't go any faster that that!
2, are you aiming to low for your target? Your BMI is great, I am currently struggling to return to 10st and you are already there! Were you truthful when you filled in the calculator on the Dukan website? If you're aiming too low you are setting yourself up for failure...