If an intro or favor has stalled, tell them there's no pressure. Lifting the guilt often prompts them to finally do it.
The idea
An ask puts weight on someone's shoulders, and obligation is the enemy of enthusiasm. When a favor stalls, the person often feels bad about it and avoids you. Releasing them frees them of that mental burden, and funny enough, it often prompts them to do the thing, because they're no longer avoiding you.
How to do it
Before you follow up, reread your original ask. Did you hand them homework?
Resist the urge to guilt them.
Send a short message taking the pressure off.
Next time, build the out into the original ask.
Example
I asked a friend to introduce me to Sarah, and a month went by with nothing. I was getting annoyed until I reread my message: "Hey, can you introduce me to Sarah? I'd love to pick her brain about X." I had just handed him homework.
Script
"Hey, just wanted to let you know no pressure on that intro we talked about. If it's not the right time, totally get it."