It's all due to a little nylon fitting inside. In the antenna channel there is a fitting with a loop that the tip goes in and a lever with a catch for the little groove in the stylus. You'd have to take the phone apart to see it. I have.
I got both of my 6700's off ebay so worries of voiding any warranty don't apply to me and I had to repair the reset button on one, which is where I came up with my fix. I put a tiny bit of foam double sticky tape under the "lever" so the catch really grabs the stylus. I did find that the Seidio pens seem to work better with the Sprint 6700's "stock" stylus dock than OEM. I have used the tape method too, as an easy fix it's probably the best (didn't like the idea of the bend the stylus method). As a general rule, I suggest NOT taking your phone apart, as it WILL void your warranty.

I have/use the Seidio Pin/Stylus for 6mo or so and it was/is tighter than the stock one. No problems with it at all, Plus it's a pen (which is certainly nice to have at times)