It's the range. You can't fly an aircraft to the extent of its range anymore. Try flying with a different aircraft with longer range.
That sounds a little silly. I have a range of nearly 4700(757-200 with winglets) and I am flying around 4350 between the two cities. What is the magic formula can I use to ensure that this does not happen on my other routes...85% of the maximum range or 80%..