Is yours leaking hydraulic oil?
I have three of these and one of them does this.
As @marjon said someone has taken his apart and found difficulty getting the "O" ring seal he needed. But he did get one in the end.
I have yet to do this with mine as I have two others that work fine.
I think, but this is just my memory, that there is some sort of a circlip at the base of the two central sliding tubes on the top of the jack body.
when I do mine I'll buy one of those O ring repair kits as they seem to work very well and can be used for all sorts of O rings.