Parker Senior Duofold c1952:Aerometric: Dry Starter

I bought this refurbished pen from an internet dealer a week or so ago and was delighted with the medium nib, with it’s smooth wet line. Suddenly, even just after filling the pen the nib is leaving no ink on the paper – this improves if I wipe the point of the nib with a tissue.

Can anyone suggest what is happening here please?

Denis