The onDrop function only fires if you are dropping something on that frame. From the sounds of the experiment you performed, you placed it in the masterindex code. If you wish for it to fire on the character sheet, then it needs to be placed within the xml for the frame that it's being dropped. As I mentioned earlier, if you are hoping to drop something from the masterindex to the character sheet and perform an action, the manager_char files in 3.5e or 5e will certainly provide excellent examples of lua checking what is being dropped onto the character sheet and branch it to the proper function to perform the tasks on the character sheet it needs to perform with the drop. If you wish to see examples of MoreCore related drag and drop you can check out Shadowrun 5e and Cyberpunk 2020 extensions as I trimmed the code from those rulesets to work with those systems.