In response to your question, it is certainly possible that you are not eating enough. Our bodies very quickly adjust to drops in calories, thinking that we might be "starving," and our metabolisms slow. However, that might not be the case, either, just possible. Keep in mind that if you are exercising, you are burning off more calories!
I've done SB from PH1 several times and my loss slows significantly. Like you, I've never had a lot to lose. One thing you might try is measuring yourself; sometimes you lose inches but the scale doesn't budge. But sometimes it just takes patience. Unfortunately.
When I am on a plateau and have looked at my menu to be sure I'm still doing everything right (because I still have to make a very conscious effort to eat veggies), sometimes I find I can get unstuck by varying my intake over the course of a week. I learned about this many years ago when I was on WW original points plan. The idea is to eat more one day, less another day, etc, to keep your body guessing.