I find the game is strange, why the train doesn't load?
http://simutrans-germany.com/files/upload/simscr02.png
Maybe the supermarket can't accept any more goods... did you check that?
I checked that the supermarket accept, and this is not the first trip.
It looks like an overloaded supermarket (check the supermarket, if it has too much supplies, it will freeze its suppliers).
If it's not that, the only possible cause is that you have modified the supermarket's station in a way it can no longer accept goods.
If this is due to an oversupplied supermarket, this is perfectly normal. Reducing the size of the train might result in a more even flow, but not necessarily better profits.