system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Can someone explain the reason why, when writing with a pen and it chokes up and doesn’t write, it still won’t write when you go over the same line again even though it will write just fine off to the margin?
gpt
When you write, in addition to adding ink to paper, you also "crush" the paper with downward force. This is why the old carbon copy graphite paper works, the force transfers through.

After the pen has run over a spot, you've crushed the paper down and created a smoothed, flat groove in that spot. If you were to look at it very closely, paper is fibrous and textured normally, but becomes flatter and more "polished" after being written on. 

Ballpoint pens require friction with the paper, to spin the ball and dispense ink. Writing over already-crushed paper is slippery, so the ball struggles to get purchase and the pen will just slide without dispensing ink.