multiply date to event

created an event, I duplicated it and changed the date.
after the description there is “event detalis”, in there, there is “additional date to the event” (or somthing like that, my plguin is not in english).
In there you can search for the additional date by the event name. Then you press the plus(+) icon to add it.
MY PROBLEM is that after I press the + icon nothing happens.

the plugin and wp are both updated