Question about Delock Cable audio jack plug > 2 x jack socket

I wanted to connect 2 Sennheiser headphones with this Delock Y-adapter. It does not work. The two Delock sockets do not make contact with the Sennheiser plugs unless I press the pair of plugs/sockets together with about 5 kg of force. Annoying, the expense of sending it back including postage.