I would be inclined to say that you are doing nothing wrong. What you lost previously you lost when you were younger. As we get older it often becomes more difficult to lose. It can also take a lot longer to start seeing results, which aren't just measured by the scales or tape-measure. There are other factors to be considered - factors like the condition of your hair/skin, the quality of your sleep, how your clothes fit, your general energy levels, etc. etc. Another thing that may be worth considering, IF you generally eat toward the lower end of the range given, you might be better off eating toward the higher end instead. I know it seems strange, but under-eating the calories can cause your metabolism to slow down. Below is a link to an article which may explain SOME of it! www.dailyspark.com/blog.asp?post=the_most_
Be patient - in time and with a little playing around to find what works for you, you WILL get there!