"Accepted drag sources" means that the drop target will accept those drag sources. It has nothing to do with whether or not they are correct answers.
You might want to make some "custom types" to simplify things.
Having 9 out of 10 possibities prevents me from assigning an action to one drag.
"Custom types" wouldn't work won't solve the problem because I want to assign an action.
The problem is solved.
Since I was not able to repro your problem, couldn't answer. Did you try to use down arrow key when you are at the last line (seems strange because 'pu' is the first one in the other screenshot)? And could you also check if all drag objects are connected to a target? Last try: change the name of that object (pu) to something a bit longer, maybe it causes a naming conflict.
No, I didn't use the down arrow key. All drag objects were connected to a target, and the change of name didn't solve the problem. It was solved by adding a text caption box (not visible) which allowed the 10th item to appear. Then, I was able to assign the actions as planned. Thank you for trying to solve the issue!