Get in a world, make a datapack with a simple function(like saying hi), then reload and try to run that function. For me, it doesn't work.
What I have to do:
Disable the datapack
Re-enable the datapack
I don't know what is with this quirkiness, but it's super confusing and annoying.
Secondly, loot tables doesn't seem to work in datapack. I tested it in a snapshot previous to 1.14 and it worked fine. Until I upgraded the world to 1.14 and it completely died away. I used the /loot command to get the loot from a loot table I made, and it seems as if it doesn't work(it does work for vanilla loot tables, but not for adjusted vanilla loot tables, where you override a loot table by making another loot table with the exact same name).
Note: If you want the datapack, comment and I'll send the datapack that I'm complaining about.
Please get in touch with the community support to have someone to give you a helping hand with data packs before reporting this as a bug here. Most likely you have just a broken data pack.