Are you sure that at the forward end of the cocking stroke, when opening the pump lever, you are pulling it all the way forward so that the breech lever (which lifts to expose the breech, carrying the transfer port with the two seals on it) is vertical?

That is to say, is the trigger mechanism and valve 'set' before you start the compression stroke of the pump lever?

I actually used mine for quite a while before realising that opening the pump lever did more than just release (and open a little) the breech lever, which I was then pulling back by hand as a completely separate action.

Regards,

Iain