Joined: Jan 2009
Posts: 44,306
Likes: 6,566
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
Hold your thumb over the opening in the head. Can you prevent yourself from pumping? If not, the plunger is not air-tight. This means rather than compressing the air, it's just going down, passing the air.
Unscrew the shaft and examine the plunger gasket. Perhaps it's dry or misshapen. Grease it on the inside and outside and try again. It may need replacement.