Squeak on joints of just installed solid wood floor

qhwang

I'm quite sure it's hardwood rubbing at joint. The strange thing is when I finish a row, they are very good by testing. But after the next row is finished, one or two joints of previous row may squeak. Now I have almost finished entire the first room. Many joints have this problem. I have #15 felt paper underneath. My subfloor is not very flat but not too bad. For some boards, I put extra very small #30 felt paper pieces. I use the Bostitch Manual Hardwood Flooring Nailer, so I use a lot of strength to hit. Is it possible when hitting very strongly, the new row has impact to the previous row and make that joint loose a bit?

qhwang

Another possible reason I came up with is the nails are too close to the end joints. I had 1" at beginning and 1.5" later. The production manual suggest 1"-3" at minimum.
Here suggest 3" at minimum.
http://www.hardwoodinstaller.com/har...s-quantity.htm