Absolute rubbish, please do not think that your blood group makes ANY DIFFERENCE AT ALL to any of the Cambridge products, I'm sorry to say that your CDC is talking a load of claptrap (to put it politely). It makes me very cross that your CDC is telling you this, you're a paying client and she is totally misguiding you.
IMO I can't see how the porridge will slow your weight loss down, it's only slightly higher in cals than the normal shakes and you're still on less than 600 cals even if you had 3 porridge a day, therefore you will lose weight. I think the only reason the porridge is slowing you down is that it's causing you to retain water, and therefore I'd drink more. Please also bear in mind that at anytime during SS you can plateau, for no obvious reason at all - I had a client not lose weight for 3 weeks and she was SSing 100%. There is no way you can't lose weight on SS and luckily she kept the faith and sure enough had an excellent loss in her 4th week. Your weight loss will fluctuate even if you had exactly the same CD every single day, you would not lose the same amount every week, and it could just be a coincidence that you had a smaller loss this week and nothing to do with the porridge. Keep looking at the bigger picture and how much you have lost in the last 4 weeks, and as long as you're averaging 10-14lbs per month, then you're bang on track.
Please don't let this put you off porridge, if you like it, have it and make sure you keep up your water intake.