You are assuming that making the "drop" sprite invisible means they don't overlap. That is incorrect. They still overlap (since they all overlap from the start). You need to destroy all of the 'other' drops when the active drop occurs, to get them out of the way. Then you will find that your Sugar & Salt drops don't work as they are ANDing. You need to break these out into a sub-event with an OR, and then perform your actions.
I have done the drop-destroy for iodine, and the OR change. Dropping iodine does nothing now. You'll have to sort out the rest.
partial fix