I'm a little confused. Are you saying your treasure boxes don't spawn at all?
As for how you add a custom item to your treasure chests...
Go into your mob_db, look for where your treasure chests. In my db they start at 1324.
1324,TREASURE_BOX1,Treasure Chest,Treasure Chest,99,0,0,0,0,0,0,0,0,0,0,0,0,0,255,0,0,0,0,0,20,0x120,0,0,0,0,0,0,0,0,0,0,0,0,732,7760,608,3000,607,2500,2504,800,2404,800,2315,800,2104,800,616,1000,0,0,0,0
Let's say your custom item is ID 4200. If you want this chest to drop it, you can change any item ID to 4200. So for example...
Change this ---> 616,1000,0,0,0,0
To this ---> 616,1000,4200,500,0,0
And that will make your chest spawn with that custom item. The first number (4200) is your item ID, the second number is the rate at which the item drops.
I hope this is what you needed!