Well downloads are not meant to do go the SD card and apps would only be able to go there if the card is made "Internal" storage instead of the standard "External" storage. This is also only possible if the phone is running at least Android "Marshmallow" and the manufacturer hasn't restricted the option. To try and turn the card into "Internal" storage, try the following... if it doesn't work, then you would only be able to move things like photos, music, and videos over to the card to make room.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded...