Roca dual flush not connected?

Hi I have a Roca dual flush, I've taken it apart as it won't flush and have found that the plunger that stops the water isn't being lifted by the lever, I reconnected them but they continue to come a part after one flush. I think there is a small cross piece, like a stopper that holds both the plunger and lever together but am unsure. If this is the problem could you tell me were to find this piece or if not what the problem could be? Thanks