I am working on creating Custom Rewards in Minecraft 1.10.2 forge-12.18.2.2114 using ChanceCubes-1.10.2-2.3.2.130. I created the JSON file originally and it would reference the name properly and display the message but would not give the item when breaking the chance cube. As a troubleshooting step I created a clean profile where it was the only mod and used a streamer reward from the sourcecode to eliminate any possibility of error on my behalf. The message still displays but still no item drops. I was able to confirm that built in rewards(rewards programmed via java not JSON) were able to drop items(coal to diamonds).

can you post a link to your json?

document.zip
this is the one i'm using, i also tried the previous 1.10 stable version of chance cubes this morning with same results