Jane:
working the lower body specifically once per week is fine and perfectly acceptable, it's what I do and you will see change, I certainly do, but remember that change is never immediate! Change takes slow, steady overload on the muscles. Working them twice per week is also fine, just ask yourself : what are you motivated to do? That's all that matters. Don't compare yourself to others, do what you feel like doing. Certainly, do not worry about it! This sounds like you are comparing yourself to some standard out there and find that your once a week is not good in comparison.
As all these posters here have said, once per week, hit the legs real hard, then after a rest day, pick up with some cardio work that is real endurance for the legs like Circuit Max, Step Works, MIC, Body Max.
I do the Circuit Max once per week too and use all the weights bits exclusively for training lower body with squats and lunges with barbell and dumbells, and forget the upper body work. I find this, some solid cardio on other days and a tough PLB or PS legs or LL once per week, is all that I want to do!
Regardless of what anyone else says or does, what will produce results for you is only what you are motivated to do. If you do legs once per week, make sure you are really exhausting the muscle, so that you wobble when you try to walk upstairs for the shower after you are done!!!!
Good luck!
Clare