Why did the schedule forecast change?
The ad-serving schedule forecast is based on the number of flights that had been started in the system when you requested the forecast. If the number of flights increased, then the forecast my change from positive to negative or vice versa.
A user requests a forecast on March 1 for a schedule from March 20-26 and the forecast is positive. By March 1, 500 flights had been started in the system for that same period.
The user requests the forecast again on March 19. By that time, 500 new flights for the period of March 20-26 had been added to the system. In accordance, there is more competition for traffic over that period and the schedule forecast changed to negative.