Loot tables

First, did you get the loot table into the chest properly? When you were holding the chest, does it display NBT+ in purple in the tooltip? If you ctrl + middle click the chest, do you get a chest with that?

If not, then you didn't put a loot table in the chest in the first place. Try using mcstacker.net to give yourself a chest, and then configure by putting your loot table where it says "loot table." It will put together a give command for you. Make sure you use the right version of mcstacker. (Also, make sure you remember to include your mod namespace when putting in the loot table.)

It is also possible that the chest does have a loot table, but you put the text in slightly incorrectly so it is not generating your loot. The solution is the same as above.

Let me know if this did not work. If it doesn't, please include your mod's namespace and the name of the loot table.