Both of these have the same cause almost every time: your survey is tied to the exact email address you pledged with, and it is being looked for somewhere else.
If the survey email never seems to have arrived
Work out which email address your Kickstarter account actually uses — it is often an older address than the one you read day to day.
Search that mailbox for the word "BackerKit", including the spam and promotions folders. The survey comes from BackerKit rather than from us, so it does not look like our other emails.
If you log in to BackerKit and it shows no surveys
A BackerKit account only shows surveys sent to its own email address. If you signed in with Google, or created the account under a different address than the one on your Kickstarter account, it will look empty even though your survey exists. Log in with the pledge email instead, or use the login link inside the survey email itself.
If you still cannot find it, we will resend it. Email [email protected] with which campaign you backed and the email address you pledged with (your backer number too, if you have it). We can check whether your survey is in and have it sent again if it is not. Please do not fill in a second survey on a different email on the assumption the first failed — one survey per pledge keeps your reward and address in one place.
Everything else about pledges — address changes, the final confirmation email, payment and refunds — is in Kickstarter, Indiegogo and BackerKit: pledges, surveys and what we can see.