That means it's still the right range for your goals.
When it doesn't change with increased fitness, and if you're on the minimum 1200-1550, this usually means that your goal date is simply too close to be realistic.
The less weight there is, the slower the body wants to lose it.
You might want to consider whether your goal is set up for 'too much' per week and think about moving your goal date back. But if you're still consistently losing at the values you have now and feeling comfortable and fine, don't change anything. :)
Deb, in New Zealand