When you lease an airplane you pay the first four month's leases up front. After four months you start paying the weekly leases. If you were barely making money at the beginning of your game, it could be that now that you have to pay the leases you just can't make ends meet any more. Many other reasons possible as well, but that's the first thing that comes to mind. The other standard answer to this much-asked question is that you have to be very careful with fleet commonality: when starting out all your aircraft should be in the same fleet group to reduce the overhead costs.