troubleshooting

Hello, When trying to thread two pieces together tight, some of my threads will sort of "slip" on each other, and no matter how much I keep engaging them, they won't stop spinning and won't lock on each other. What can cause such a problem and how would you solve it? My threads are made from...

A young person of my acquaintance was assembling his first-ever pen (slimline), and he pushed the transmission in too far by just a half mm. Now, even when the pen is fully 'retracted,' the tip of the refill sticks out past the tip of the pen. Is there any way to back the transmission up...