What Powi says is correct. But I don't think that's the heart of the problem.
The issue to me would be your route choices. Yes, the majority of your routes are without competition. But, your avg route is very long for a T-prop.
I ask, how many flights per day does your DC-7s and 6s avg? 2.. maybe 3 from what I can tell. What if I told you my viscounts avg 4-5 routes per day. Thus, my profit potential is nearly double yours.
Sure, you make a profit flying 2 1000nm routes. But you can make twice that much flying four 250nm routes.
This is the reason your stalling. You had success in the beginning w/o paying leases. Four months later, that "extra" money is gone and you dont make enough to pay the bills. The reason you dont bring in enough income is because your T-props are flying long routes. Limiting the amount of routes you fly hurts the aircraft's profit potential.
In domestic/short haul international flying, it takes roughly 2 flights to "break even". Depending of course on the aircraft. Breaking even does not
mean the little info that shows the "profit" an aircraft is making. It means paying for leases, fuel, fees, labor fleet common marketing ect. All
your companies expenses.
After 2 routes, that generally turns into profit straight to the bottem line. Again, this all depends the aircraft you fly. So are better then others.
T-props are excellent up to about 400nm or so. After that, the lack of speed adds to flight time which then limits the amount of routes you can stuff into the aircraft.
More routes = greater profit potential. Generally, four routes will make more money then two routes. This really applies to domestic/short haul international were ticket prices are low and flying further only nets a few extra dollars per seat.
Longhaul is a different story, for another day
Hopefully this helps