Drive In Grease Fittings

[fundytides]

I decided to change all of my grease fittings over to the modern style. There was a mix of original and modern on the car. I am having trouble with the new drive in fittings. I am trying to drive them in with a small socket on an extension and a hammer. They won't seem to go. Do I need to enlarge the holes a bit or just use a bigger hammer? Anyone else run into this?