Losing weight requires you achieve a calorie deficit (in other words, burn more calories than you consume each day). There's two sides to this equation - Calories In, and Calories Out.
What worked for you for weight loss at 198 is probably no longer sufficient at 161, because your basal metabolic rate is less now (a 161 lb body has to burn less calories than a 198 lb body).
Your choices are: exercise more, eat less, or do some of both.
You may want to get your body fat percentage checked at a local health club or university (or, you can get an estimate from this website
http://www.he.net/%7Ezone/prothd2.html ).
Also, you might want to check your US weight percentile, using this web page:
http://www.halls.md/body-mass-index/av.htm