I can only echo what others have said - you may simply be eating too little food for your particular body and metabolism. Cambridge, like any other diet (including my beloved Atkins) does not 'work' in the very same way for everyone. Our bodies try to save our lives; they think we are eating 500 calories a day because that is as much food as we can find. The body has no clue we are 'starving' on purpose. So, to keep us alive, heart beating, breathing, etc for the longest time possible it slows down the rate at which we burn fuel - i.e. - it lowers our metabolic rate.
I must say here that many on this site, on VLCDs, will disagree with me. There is a common belief that TFR diets 'protect' us from starvation due to their high nutritional content. Not so. The generous protein levels help to minimise, but not totally prevent, the muscle loss that accompanies fat loss. And, technically, the body is not being 'starved' of the bare minimum of nutrients needed for reasonably healthy survival. Nonetheless I believe that such a low daily intake, over a long period of time, has a significant slowing effect on metabolic rate. I don't see how that can be avoided. Regardless of the nutritional value of the packs, they total around 500 calories per day which is of course way too low for 'normal' life.
I first did Cambridge 20 years ago and, again, in 2007. I found that I needed far fewer calories afterwards to maintain without regaining FAST and pretty soon I had to exercise like a mad thing every single day. It took ages for me to be able to eat more than 1000 calories a day without seeing real (not water) regular regains on the scales, week by week.
I dealt with it by increasing calories very very slowly and by cutting out most sugars and starches. I doubt I will ever be able to eat generous amounts of any kind of food but I know for a fact that for me the only way to maintain is to low carb.
If losses stall or slow for weeks on end then it may be time to move up a step or two. Even on Atkins people stall or slow when they eat too little!
