Sorry for not replying to you earlier.
Since a recent patch, trucks and train wagon can only transport one cargo type. You can define the cargo type when buying the vehicle or while it is in the depot. While buying, click the icon next to the capacity in the right hand side of the window to change the cargo type. In the depot, you can click on the cargo symbol which overlays the vehicle.
If you want to get rid of this restriction, you can use a mod provided by us:
Just extract the zip file to your mods folder in the Train Fever directory and activate it when starting a new game.
It is available in German and English. When the mod is installed and active, vehicles can carry all cargo types.
If you have any further questions, please let me know.