A common but preventable reason for impression rejections are 'drag marks' and 'scrapes'. That's where your teeth scrape into the putty. Sometimes these marks are created if instead of biting straight down into the soft impression material in the trays, you slide your teeth into the putty a bit sideways or you move the teeth while the putty is still soft. 

The marks can also happen if you remove the trays before the full 3 1/2 minutes are up. If the putty is still a bit soft, you can create these marks when you remove the trays, especially if you don't pull straight down. Always use a timer and keep the trays in for the full amount of time. If you happen to forget to turn on the timer, then still do 3 1/2 minutes once you remember. It's better to leave the trays in too long, than not long enough. (Most people over-estimate the time the trays have been in their mouths).