Kitchen Tap Problem

[Lulu1945]

Our kitchen tap has become very stiff in the last couple of days. My Husband has taken off the top ( it is a single control on the top with a swinging arm below). There is a square plastic protrusion which houses the tap top when screwed in position, and which moves backwards and forwards when the water is turned on. There does not appear to be anything that can be adjusted though we have sprayed WD40 to see if that helps but it hasn't. In fact it is stiffer than ever.
Is there anything below the Tap top that can assist in loosening the Tap?

 

[John506]

Hard to tell which tap you are talking about from description, can you post a picture, or a picture of the same tap type from google will do.

 

[Lulu1945]

Thanks for getting back to me.
It has 'Oceana' and 'Group Roca' written on the top.
Purchased in France. It was an expensive tap, by French standards
otherwise I don't know the model.