mechanical pencil and mechanical engineering

I was happily taking down notes with my Pentel PG305 mechanical pencil when it ran out of lead. So I took out the cap at the back, inserted a new piece of Mitsubishi Uni 0.5mm lead, and clicked away.

After what seemed like eternity of clicking, there was still no lead coming out from the tip. So I took out the cap again, tilted the pencil, and to my surprise, there was no lead inside! Where did it go???

Some mechanical problem can't be solved. Or, I solved it by inserting a new piece of lead.
(and you won't deny that mechanical pencil is one of the greatest mechanical devices invented)