It could be going into junk/spam folder (or in Promo tab if they are Gmail users).
Did you happen to change up the content of the incentive email? If so, then maybe inadvertently the content triggered it. I've had it happen myself before.
Keep it simple and direct to the point of clicking the button to download.
Avoid using words like "free" "access" and other potential spam trigger words.
To be certain, test your incentive email by using a tool such as Mail Tester8