Barb offered up some good information. Another thing I would try is altering your diet in terms of eating times. If you normally eat 3 meals a day, eat 5 small meals with some protein on either side of your work out.
Also, if you aren't doing it... adding weight training would be great. I'd do a light warm up on the treadmill, then hit the weights, then pour on the cardio. Change your cardio routine a bit too.
Change up your calorie in take and exercise routine.