Thanks a lot - indeed, I missed the too small warning (was too focused to fix the slots). I wasn't expecting it as the route is well within the range of the plane.
What would you advise me to do? Go back to a larger plane?
Does this mean this can be happening on the 4000nm+ routes with B737ER?
The A310 will probably be the best fit for you on that route - you've got to go larger. The 757-200 that your competition is flying is not too small (yet, but may be later in the game).
So, for the 737ER, you can do some forum searching as I've typed this out countless times, but basically the too small warning depends on the total DEMAND of the route, the DISTANCE of the route and the game year.
So, for example, in a given year, you may see the following for a 737ER:
125 demand - No warning
200 demand - Warning
Now, 30 hypothetical years before the above example, both might be fine and receive no warning.