Forecasting with a Machine Learning Algorithm
I'm afraid your data is probably too complex and specific for somebody else to understand exactly what's going on.

The only idea I can suggest is to try to analyze manually the errors that your model makes: 


Are there any patterns, like a kind of errors which happens quite often? For instance a country which tends to be overestimated, a month which tends to be under-estimated, this kind of thing.
If yes try to investigate why: for instance is there some additional feature that could help the model make better predictions for these cases?