weight loss

Have been following a weight watchers diet religiously, even down to drinking 2litres of water a day, for the past month.
So far I have managed to loose 3/4 of a pound. I am 61 years of age and have an office/floor walker job in Inspection and do a half hours Exercise each evening.
My question is could there be some form of Medical reason why the weight loss is so little, I have no medical problems at the time of writing.