I have gone through several stages in my brewing process. Following what my friends told me in terms of the German Purity Laws, I used only DME for quite some time. One day I joined the dark side and used corn sugar for priming, and frankly, I never looked back. It's always better (for me), carbs quickly (2 weeks or less) and tastes great. When I would use DME, there would be more sediment, and carbonation was always longer (but it would eventually carb, just not as strong).
Hang tight, your beer will carbonate, it just may take an extra week or two. When I used DME, it was a minimum of 3 weeks before drinking, and frankly longer was better. Some though would never catch up. 1.25 cups is good for the average beer (for DME). If you use corn sugar, I believe it's around 2/3 - 3/4 cup per batch. Works well for me every time.